[ENG] The troubles of creating a disc-drum combined braking system is a lot more than you might think

Starting in the 1960s, the front disc and rear drum brake combination braking systems started becoming very popular, especially in Europe. In the USA they were still only reserved for high performance and expensive cars, however in Europe, they became very common in the 1960s and soldiered onto the 1970s and onwards. In this post, we will take a look at each drum and disc brakes separately, then look at the challenges of creating a disc-drum braking system and explore some safety features fitted to these systems that you might not be aware of.

Autor: TAUNUS

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The differences between disc and drum brakes

There are lots of differences between disc and drum brakes not only in their operation about how they create friction to slow the vehicle down, but also their actuation methods are very different. This means that combining both systems also brings a lot of challenges with it which we will take a look at after first analyzing each system separately.

One of the biggest differences in the operation of drum brakes as opposed to disc brakes is the fact that drum brakes are self energizing. This means that when drum brakes are engaged by pushing the brake pedal, the self energization phenomenon of drum brakes itself will also start aiding the force of the driver on the brake pedal and amplify the force.

Thanks to this self energizing operation, drum brakes generally do not require a lot of force by the driver to stop the vehicle, hence why a lot of full drum brake cars in the past majority of the time did not have a brake booster servo.

Disc brakes, on the other hand, are not self energizing, and they require a lot of force by the driver to generate enough fluid force in the brakes to create frictional force in the disc brakes. This is why disc brake cars generally came with a brake booster servo, which does the job of amplifying the force exerted by the driver on the brake pedal.

The self energizing nature of the drum brakes also bring a downside which is worse feedback feel to the driver, which ends up in driver having a hard time finding the optimum braking force to stop the car as fast as possible without locking the brakes.

Another big difference is that the brake pads in a disc brake setup always slightly rub against the surface of the disc. This happens due to the different release mechanism utilized by disc brakes which will be explained in disc brake section.

This in turn creates some parasitic drag on the car. However, this drag is tiny and not noticeable and also does not really cause any extra wear on the rotors and pads.

Some drag racers use drum brakes on all corners because drum brake’s shoes are never in contact with the brake drum when the brakes are not applied so they don’t have parasitic drag, and they are lighter than disc brakes. While this can matter in drag racing where every millisecond counts, the parasitic drag will never be noticeable on the road.

Drum Brakes

Drum brakes utilize a „brake drum” and a „backplate assembly” which, when both are combined together, creates the drum brake mechanism. The brake drum is extremely simple, it is a drum shaped metal that has inner running surfaces to create friction. This brake drum attaches to the wheel hub and always rotates with the wheel, and it can be seen in the picture below as the component on the right.

The backing plate bolts to the axle housing itself or to the steering knuckle if it is a front drum brake, so the backing plate is always stationary. All the rest of the components inside a drum brake reside in this backing plate.

The backing plate contains brake shoes, wheel cylinder, return spring, hold down spring, self adjustment mechanism and the handbrake mechanism if it is a rear drum brake. In simplest terms, the drum brake works when the hydraulic fluid exerts pressure to the pistons inside the wheel cylinder which pushes both of the brake shoes towards the inner surface of the brake drum and with the friction material lining on the brake shoes, it will grab hard onto the brake drum and generate a lot of friction to slow down the vehicle.

When the driver stops pushing the brake pedal and exerting pressure on the pistons inside the wheel cylinder, the return spring will force the brake shoes to return to their original position where they won’t rub against the inner surface of the brake drum.

Types of Drum Brakes

Simplex Drum Brakes

There are a couple of different drum brake versions, some of which can be seen in the picture above. The simplest is known as the „Simplex” in German and as „Leading/Trailing” in English. In this type of system, one of the brake shoes is the leading shoe shown as „1” and the other one shown as „2” is the trailing shoe in the picture below. There is a double acting wheel cylinder located at the top between the brake shoes which can be seen as the location of the arrow in the picture below.

Like in the picture above, if the car is going forward then the wheels will be moving counterclockwise if we view the car from the left side.

Whenever the wheel cylinder pushes out both of the shoes, the leading shoe on the left will try to be dragged in the CCW direction by the CCW rotating brake drum by a tangential force. However, both brake shoes are anchored at the bottom, so it can’t physically move in the direction of the brake drum.

The CCW rotating brake drum with tangential force will still try to drag the leading shoe with it which will cause the leading shoe to bite harder and this is what causes the „self energization” effect. The force exerted by the driver pushing the brake pedal is amplified in the leading shoe.

In the trailing shoe however, the opposite is true. In the trailing shoe there is self de-energization because on the trailing shoe the rotation of the brake drum with tangential force tries push the trailing shoe away from the brake drum and put an opposing force on the pressure coming from the wheel cylinder.

However, the amount of self energization of the leading shoe is higher than the self de energization taking place in the trailing shoe so that overall there is still force amplification via self energization.

This also means that if the wheel rotated in opposite CW direction lets say when reversing, the leading shoe now will be the right shoe and the trailing shoe will be the left shoe and the same amount of self energization will take place but in the opposite brake shoes due to opposite direction of rotation. This can be seen in the diagram below, along with all the forces acting on the whole assembly when braking.

The simplex type of drum brake is generally used on the rear brakes unless the car is rear engined like a VW Käfer where the rear brakes also do a lot of work. On front-engined cars with front disc brakes, the rear drum brakes are generally of the simplex type which works great in that application with not much self energization where rear brakes are not doing much, and the parking brake is built into them.

However, 4 wheel drum cars generally tend to use duo-servo type of drum brake at each corner because they have a much greater self energization effect, which will be explained in detail in duo-servo section. Using duo-servo up front and simplex at the rear will cause a great imbalance between braking forces due to duo servo generating a lot higher braking forces than the simplex drum brake, so simplex rear drums are generally used with front disc brakes.

In the simplex drum, the parking brake will also take advantage of this self energization effect and provide a strong force in the drum brake to hold the vehicle stationary.

Additionally, the self energization effect is hard to control, the brake pedal might feel very grabby because the more you push, the more self energization there is, so it’s much harder to brake at the limit of grip with drum brakes compared to disc brakes. This is why disc brake cars in the 60s and 70s braked in shorter distances than full drum brake cars because the driver could manipulate the brake pedal better in the disc brake cars.

There is a very big misconception that disc brakes provide more force than drums and hence the car stops faster. This is completely false, there is not enough grip in the tires in the first place, so more braking force doesn’t mean anything. Also, with a drum and a disc brake of equal sizes, the drum brake will actually provide even more braking force because the drum brakes friction area is at the circumference of the brake drum so it has more distance from the wheel center to create higher braking torque.

The reason disc brake cars stop better came down to the fact that they can be controlled easier by the driver without an ABS system.

In the rain as well, discs have an advantage because they can displace the water immediately but when the water gets inside the drums, it basically soaks them and then drum brakes cannot generate enough braking force, so disc brakes achieve an even bigger advantage over drum brakes in the rain. However, in dry situations, drum brakes provide a lot of braking force.

Duplex and Duo-Duplex Drum Brakes

The second type of drum brake is the Duplex and Duo-Duplex systems. In the Duplex system, there are 2 leading brake shoes instead of one leading one trailing shoe construction of Simplex brakes. This means that there is self energization in both of the brake shoes which provides a lot more force amplification compared to simplex brakes.

As it can be seen in the diagram above, there are 2 wheel cylinders. One of them is located upwards, and the second one is located at the bottom of the brake shoes. On the regular Duplex system the wheel cylinders are not double acting, they only act in one direction.

This means they only have self energization in forward direction. While they can act as leading shoes in forward direction, they cannot do this in reverse direction due to wheel cylinders only pushing them one way, so they have self de-energization in reverse and brake pedal gets very stiff in reverse.

In the Duo-Duplex system, both wheel cylinders are double acting like it was on the simplex brake. Combined with the 2 wheel cylinders in total, these brakes achieve complete symmetry in actuation of its brake shoes in either direction. This means that in the Duo-Duplex system there is great self energization in both forward and reverse driving situations giving a soft brake pedal.

Dual Trailing Shoe Drum Brakes

Third type of drum brake is the dual trailing shoe system where it is exactly same as the Duplex system however the direction where the single acting wheel cylinders act is reversed so that both shoes act as trailing shoes against the direction of rotation of the brake drum as it can be seen in the schematic below.

This means when driving forward there is a lot of de-energization and in reverse there is a lot of energization, the complete opposite of Duplex system. This system is used when the precision of the brake pedal is important without the self energization effect making the brake pedal very hard to judge by the driver. These systems are used with brake booster servos to lighten the brake pedal just like disc brake systems. This system is also not common at all.

Servo and Duo-Servo Drum Brakes

The fourth type of drum brake system is the Servo and Duo-Servo systems. These types of brakes are also referred to as „Bendix” brakes because they were originally designed by Bendix. They are also called as „Primary/Secondary” drum brakes. These types of brakes are very common on the front wheels due to how much self generation they can create. They are also used on the rear brakes of rear engined cars such as VW Käfer and Porsche 356 because the rear brakes on those cars do a lot of work.

As stated before in the simplex section, all around drum brake cars also use duo-servo brakes at the rear and generally only front disc rear drum brake cars use simplex style drum brakes at the rear.

These are very similar to Simplex brakes in construction, especially the Duo-Servo. The single servo utilizes a single acting wheel cylinder at the top between 2 brake shoes and the Duo-servo utilizes a dual acting wheel cylinder in the same location, just like the Simplex drum.

Where they differ compared to the Simplex drum is in the lower section. In the simplex drums there is an anchor at the bottom of the brake shoes that fixes the lower section of the brake shoes in place, preventing them from moving.

In the Servo systems there is no anchor, instead the bottom parts of the brake shoes are floating and are connected via the star wheel of self adjustment mechanism, so they can freely move around. Recall that in Simplex drum, whenever the wheel cylinder pushes out both of the shoes, the leading shoe will try to be dragged downwards by the counterclockwise rotation of the brake drum but couldn’t move due to the brake shoe being anchored into place.

In the servo systems, because they are floating, they will actually move together with the brake drum. In the servo system, the „leading shoe” is called as the” primary shoe” and the primary shoe gets dragged a bit by the brake drum in the same direction that brake drum rotates.

The other brake shoe is called as the secondary shoe, and it is connected with the primary shoe at the bottom via the self adjustment mechanism. So when the primary shoe moves when the brake drum drags it in the same direction, the primary shoe will push the secondary shoe in the same direction the drum rotates.

Again, imagine the scenario of looking on the left side of a car and the wheel is rotating CCW. When the brakes are applied, the brake drum will pull the primary shoe downwards and this motion will cause the secondary shoe to move upwards towards the counterclockwise rotation of the brake drum. This phenomenon can be seen in this video.

When this happens, the secondary shoe receives force not only from the wheel cylinder, but also from the primary shoe and overall, the forces will act very symmetrical on it and will grab onto the brake drum very hard.

In the servo system, the primary shoe is overall not doing much, unlike in the simplex system. Majority of the braking force in servo systems is generated in the secondary shoe and the main purpose of the primary shoe is to be dragged by the brake drum and this way exert a lot of force on the secondary shoe so that secondary shoe can create a lot of friction with the brake drum. While the primary shoe also wedges into the rotating brake drum and creates its own self energization, its main purpose is to push the secondary shoe.

This also means that secondary shoe wears down a lot faster than the primary shoe so the secondary shoes generally have several times thicker friction lining material compared to primary shoes.

The difference between single servo and duo-servo is that duo-servo with its dual acting wheel cylinder can self energize in both directions, meanwhile single servo system can only self energize in forward direction.

The self energizing effect achieved via servo brakes is a lot higher than in simplex system so they are generally used at every wheel in pure drum brake cars so that a lot of braking force can be generated and reduce pedal effort without needing a brake booster servo. The diagram below shows the differences in pedal effort between duo-servo, twin leading shoe and simplex drum brakes.

However, this greater self energization also means that they are even more sensitive to control by the driver, and they are also very sensitive to the shoe lining properties caused by heat and wetness.

Drum Brake Release and Self Adjustment Mechanism

The way drum brakes are disengaged is by the return springs attached to the brake shoes. When the driver lets go of the brake pedal, the hydraulic pressure is no longer exerted on the wheel cylinder and the return spring pulls the brake shoes back into their parking position.This method is very different from how disc brakes deal with this situation.

The return springs give a more positive brake release feel to the driver, but also have the downside of having some lag. Whenever the driver presses the brake pedal, the hydraulic pressure first has to overcome the return spring tension, so there is some small amount of delay between when driver presses the brake pedal and when brake shoes start moving. This phenomenon has critical importance on disc-drum brake systems, which will be explained later.

As the brakes are used, the friction lining material also becomes thinner and thinner and the driver would have to push the pedal more to achieve the same braking force. To combat this, a self adjustment mechanism is fitted to most drum brakes. The exact operation of the self adjusting mechanism is complex, so directly watching this video will be more helpful than reading the text.

The self adjusting mechanism used in drum brakes is a complex mechanical system, in the disc brakes this mechanism is also much simpler and completely different and functions as both the pad return mechanism and self adjusting mechanism.

It should also be noted that the self adjustment mechanism in drum brakes is not very reliable and many times they do not work, and you will still have to adjust the drum brakes manually.

This is due to there being too many debris inside the drum from brake dust and other particles and because the self adjustment mechanism will only work when brakes are used in reverse or in some cars when the parking brake is engaged. This means that on a race car, for example, this system will be completely useless and never automatically adjust the shoes as the friction lining wears down.

The self adjusting system used in disc brakes, which will be explained later, is extremely reliable, so as a driver you mostly never have to adjust your disc brakes.

One final note about drum brakes, the brake drums can be made of aluminum which dissipates heat faster than cast iron and this was a great method to combat brake fade with drum brakes. Another great and more cost friendly solution was putting cooling fins on the brake drums, as it can be below.

Fins and aluminum in combination was a great method to combat brake fade and was the method used on racecars using drum brakes. If designed correctly together with the wheels, more air can be forced to flow between the fins of the drum brake and provide an even greater cooling which can be seen in the W198 diagram above how the rotating wheels force air through the finned drum brakes.

Many performance and sports cars back in the 50s also used aluminum finned drums, such as Porsche 356 and Mercedes W198 300 SL. In 1958, Buick also adopted aluminum finned front drum brakes on their cars. One more advantage offered by aluminum drums was they decreased the unsprung weight a lot as well, which helped in comfort and handling.

While disc brakes still offer superior cooling compared to aluminum finned drums, they were a great improvement over standard cast iron non finned drum brakes used on regular cars.

It should also be noted that the friction surface on the inside of the aluminum drums were cast iron sleeves because aluminum has very bad wear resistance properties so cast iron sleeves were still used on the friction surfaces and the rest of the brake drum was aluminum for much better heat dissipation.

Finned drums were also sometimes utilized as rear brakes on front disc rear drum brake cars such as Mercedes W110 and Audi 100 C1 to provide more cooling capacity to their rear drum brakes, which could be useful in a mountain descent. In my opinion, nothing beats the finned drum brakes in terms of looks.

Overall, drum brakes are critical in their adjustment and maintenance, if wrong shoes are used or if they are not adjusted properly, it can really mess with their function. If maintained properly, they can still be very useful in dry conditions and in conditions that do not require frequent high speed stops because drum brakes cannot dissipate heat fast enough and will experience brake fade much faster compared to disc brakes.

Disc Brakes

Disc brakes are much less complex than drum brakes and are also much easier to maintain. They barely get affected by water, they have much better cooling, they are easier to control. However, they lack self energization so they require a brake booster servo, otherwise the brake pedal will require a lot of force by the driver.

In the disc braking system there is a brake rotor/disc and a caliper which houses brake pads. The rotor is mounted to the wheel hub and the caliper is mounted to the steering knuckle or to a suspension link. The caliper is placed in a way that will surround the disc from both of its surfaces, which can be seen in the picture below.

The disc brake works by the pistons inside the caliper receiving hydraulic pressure and being pushed. When the pistons are pushed, they, in return, push the brake pads onto the rotor that is rotating with the wheel hub, so the brake pads squeeze the rotor. When both pad and rotors contact, friction is generated, which slows down the vehicle.

Types of Disc Brakes

There are 2 different popular types of disc brake systems. The most common one is the floating caliper system and the less common one is the fixed caliper system.

Floating Caliper Disc Brakes

In the floating caliper system, the piston or pistons are located only on the inner side of the caliper. The outer side of the caliper does not have any pistons in it and the whole outer section can laterally slide.

As it can be seen in the schematic below, the hydraulic fluid comes and pushes the piston on the inner side of the caliper which will engage the inner brake pads with the disc. Until the piston moves quite a bit and the inner brake pads start applying pressure on the disc, the outer sliding section of the caliper will be stationary.

Once the piston has extended quite a bit and cannot easily move out further, the hydraulic fluid will now start applying pressure to the inner wall of the caliper frame that holds the outer sliding caliper. When this happens, the outer sliding section of the caliper also starts moving and the brake pads attached to it will come into contact with the disc. This can be visualized better at timestamp in this video.

This means that the inner caliper always puts pressure on the disc first and then the outer caliper starts applying pressure on this disc, which means the pressure application on the disc is uneven. This is not a problem for most road cars, but it does become a problem on performance cars in regard to brake feel and performance.

The floating calipers can have more than just one piston, but the pistons will always be on the inner caliper and never on the outer sliding section. But generally on street cars you will find single piston floating calipers.

The amount of required pistons in a braking system depends on the aspect ratio, which is the ratio of brake pad circumferential length to radial width. So the bigger the brake system gets, so does the brake pads themselves and to apply even pressure on the pads, more pistons are required.

Another advantage of floating caliper is that the handbrake mechanism can be easily integrated into it without much complexity or cost. As it can be seen below, there is a camshaft connected to handbrake cable.

When the cable is pulled, the camshaft rotates and this causes the camshaft to push the threaded shaft. The threaded shaft then pushes the piston onto the face of the disc and at the same time, the outer caliper also begins to slide and starts grabbing the disc as well and both inner and outer caliper sections put pressure on the disc and prevent it from rotating.

Fixed Caliper Disc Brakes

The other common type of caliper system is the fixed caliper, which was very common in the old days, even on normal cars, but these days they are mainly used by performance cars.

As the name implies, the caliper is completely fixed and does not have a sliding section. This means that both the inner and outer parts of the caliper must have a piston. So the least amount of pistons on a fixed caliper will be a 2 piston design where there will be a single piston on each side of the caliper that will oppose each other as it can be seen in schematic below.

Of course, the amount of pistons will also increase with increasing caliper size, for example, the current 992 generation of 911 Turbo S has a huge 10 piston fixed caliper which houses 5 pistons on each side of the caliper.

The caliper has a hydraulic channel for hydraulic fluid to pass inside the caliper and reach the outer side piston. Both pistons receive the fluid at the same time and both move at the same time and rate, which provides a very even application of the caliper over the rotor.

Fixed calipers offer many advantages over the floating style calipers. The fixed calipers are a lot stiffer and beefier than floating calipers and this means that they are very durable for use under high fluid pressures and can dissipate heat more quickly than floating calipers, which means it will be harder for brake fluid to boil when using fixed calipers.

The stiffness also helps a lot when the temperatures rise, the floating calipers tend to flex a lot and their consistency and feel degrade even further when they are under a lot of heat. It should be noted that the majority of fixed calipers are 2 pieces design, which still have a somewhat compromised rigidity while still being a lot better than floating calipers.

The stiffest fixed caliper pistons are known as „monoblock” calipers and they are a single much stiffer piece that are even stiffer than the 2 piece fixed caliper design so they are the preferred choice when it comes to racing applications meanwhile street performance cars will use a 2 piece design.

This extra stiffness of fixed calipers, combined with the fact that they heat the brake fluid less, makes them a much better for use in a performance car where brakes get very hot and consistency and feel under these hard circumstances is very important.

However, they are also a lot harder and expensive to manufacture than floating calipers, harder to service, and they are also heavier, which hurts the unsprung weight which is very important.

Of course, this last point can be countered by using inboard disc brake systems where calipers will be mounted to the frame or subframe so that their weight will become a sprung weight and not an unsprung weight. However, inboard disc brakes also completely disappeared in the 21st century, and they were not very popular in the 20th century either.

Another huge disadvantage is their size, they take up much more space inside the wheel, which can make it very hard to achieve certain types of steering geometries, this is another big reason why most cars today switched to floating calipers. This is not a problem on race cars on performance cars with very wide track widths, but it is a problem on normal road cars.

Yet another disadvantage of fixed calipers is they cannot be used as a handbrake like floating caliper disc brakes can be. For this reason, on cars with rear fixed caliper brakes, an additional drum brake is used solely to serve the purpose of a parking brake.

Below, the handbrake setup from Ferrari 288 GTO can be seen. The brake shoes sit inside the disc and they when the handbrake is pulled, the brake shoes will be expanded into contact with the inner surface of the disc to provide friction. This is a much more costly and complex option for having a hand brake.

Disc Brake Release and Self Adjustment Mechanism

The exact application and release system of the disc brakes is interesting and not at all like the drum brakes with return springs. Let’s start by taking a look at the floating caliper system. The caliper has a machined groove inside it where a flexible rectangular cut o-ring seal slides into. This o-ring seal is always in contact with the piston inside the caliper, as it can be seen below.

Whenever the hydraulic fluid starts pushing the piston, this flexible seal also flexes and follows the movement of the piston by flexing because it tightly grips the piston. When the hydraulic fluid no longer exerts a force on the piston, the flexible seal now springs back into its original shape while also still grabbing the piston, which means it will also pull the piston back from the rotors and this way achieve a returning mechanism.

This can be seen in the schematic below. When at rest, the seal has not flexed like in the example shown in „1”, however when the piston moves, the seal flexes and takes the shape shown in „2”. Once the hydraulic force on the piston is removed, the seal goes back to its resting state. This video also does a great visual representation of what is happening.

Some very old disc brake systems such as the ones from 1960s MG sports cars still utilized return springs, but they quickly disappeared after it was realized that the seal provided a good returning mechanism for the piston.

This seal system provides very fast direct response when the driver depresses the brake pedal, there is no time spent to first overcome the tension of the return spring to apply the brakes. Recall that as friction lining material wears on brake shoes, they needed adjustment to keep the same working distance and pedal travel and how this was done with a mechanical self adjusting mechanism on drum brakes.

On disc brakes, there is no such mechanism. Instead, this same seal system also does a very good job of self adjusting the brakes. In the floating caliper system, once the clearance between brake pads and the rotor becomes too much, the square cut seal will let to of the piston and slide over it.

This happens when the brakes are out of adjustment and driver presses the pedal, the piston will now travel an extra length and the seal cannot flex after a certain amount. This means that if the piston has to travel too far, the seal will not be able to flex anymore and won’t be able to hold on to the piston, so the piston will slip through the seal and push itself further out and stops once the brake pads again get into contact with the rotors.

After this happens, the seal grabs a hold of the piston again and once the hydraulic pressure is removed, it brings the piston back just a touch, the same amount that it flexed. Now the piston has progressed further towards the rotor and took up the slack automatically. This system is very reliable and never fails unless the seal has failed, which takes a very long time, unlike in drum brakes where even brand new those systems did not work very well.

This is the reason why disc brakes do not use return springs. The seal mechanism worked so much better than both the return spring and the mechanical self adjustment mechanism and a single part managed to replace 2 mechanical more expensive parts while doing their job in a much better way.

However, this seal system also has one downside, they cannot fully retract the pistons back. They do a very good job of returning them back into their parking position, but the seal alone cannot provide 100% enough force to completely return the piston back.

The pistons do not really put a pressure on the rotor, the seal has done enough of a job to prevent that, but the pads will still be lightly resting on the surface of the disc and always rub against it. This means that disc brake systems actually create a parasitic drag on the whole drivetrain which will reduce fuel mileage, power going to the ground and increase emissions.

It was due to this reason, low drag seals were invented and put into use in the 1980s. These low drag seals help increase the clearance between the pad and the rotor to reduce parasitic drag losses.

In these calipers, the groove where the seal fits into has a tapered outer edge.This tapered edge allows the seal to flex even more, which stores more energy in the seal so that when it springs back, it will grab the piston harder to get the pads off from the rotor. This is still not a perfect system, so even in this there is still some drag but a lot less compared to regular systems.

These low drag systems also require a special master cylinder known as a „quick takeup master cylinder”. These master cylinders provide a greater volume of brake fluid with the first part of the pedal travel to take up the excess pad to rotor clearance quickly so that the master cylinder normal brake pedal height and travel and also be able to apply the brakes still quickly, so they won’t have extra delay compared to regular system.

This square cut seal also has another purpose, which is preventing the hydraulic fluid from leaking out of the caliper. There is another additional dust seal located further ahead of the square cut o-ring seal which prevents dust and debris from getting inside the caliper where it could damage the square cut seal.

On fixed caliper brakes, the same type of seal system is used, however it is arranged a bit more differently. The seals on fixed calipers are called as „stroking seals” and they do not go inside a machine groove inside the caliper.

Instead, these seals go directly on the piston itself and moves along with the piston itself, it is not stationary like in the floating caliper design. However, the way it works is the same, once the hydraulic fluid pushes the piston, the seal will flex and continue with the piston while the other end of the seal will grab onto the walls of the caliper as it can be seen in the schematic below.

Once the hydraulic fluid pressure is removed from the piston, the seal will want to spring back the same way and bring the piston back and the pads away from the rotor. In these systems as well, the pads will lightly rub against the rotor and create parasitic drag.

The self adjustment system is also very similar. Recall that in the floating caliper the piston will move out and slide over the stationary seal. In the fixed calipers, the seal and piston will move together towards the pads to take up the clearance and the seal will slide over the caliper walls and move together with the piston itself.

Short Summary of Drum versus Disc Braking Systems

As a recap of drum versus disc braking systems, drum brakes, due to their friction surface being on the circumference of the drum, create more braking force compared to discs. Drum brakes have self energization so they don’t have to use a brake booster servo, but at the same time, this self energization makes them a lot more sensitive and harder to use.

Drum brakes have very poor self adjustment mechanisms, so the owner of the car still very frequently has to adjust them, and drums are very sensitive to misalignment and will drop heavily in performance and will pull to one side if they aren’t adjusted correctly.

Drums experience brake fade a lot quicker and also recover a lot later than disc brakes due to being an enclosed design. Drum brakes cannot displace water due to the same enclosed design and work very poorly in wet roads.

Drum brakes utilize return springs which create a delay between when driver presses the pedal and the brake shoes expand into the drum, disc brakes with their seals are instant.

Drum brakes do not create any parasitic drag losses on the vehicle meanwhile disc brakes do, for this reason as well, disc brakes can apply faster than drum brakes. The lack of return springs is the main reason, but the pads resting lightly on the surface of the rotor in disc brakes also means the pads do not have to cover any gap to create friction, it will happen instantly and faster than in drum brakes.

During the 1960s, front disc rear drum brake systems started becoming very popular in Europe and also in some performance and luxury American cars thanks to reasons like disc brakes never needing any adjustment and having much better performance in wet along with much superior cooling capacity to prevent brake fade.

Since the majority of the braking is done by the front wheels on front engined cars which were and still are the norm, the rear brakes were not of great importance so they were left as drum brakes during the 60s and 70s except in rear engined sports cars such as Porsche 911 that used disc brakes both in front and rear wheels.

The challenge of combining front disc and rear drum brakes

Recall that drum brakes take quite a bit longer time to engage compared to disc brakes, this means that if the brake pedal is pressed on a front disc rear drum brake car which also does not have a metering block, the front discs will apply so much sooner than the rear brakes and due to the only front brakes being applied, the car will take a huge nosedive and the rear brakes will never be able to catch up.

This will not only be very unpleasant for the passengers, it will also lift most of the weight off the rear of the car and the rear brakes will become nearly useless. This is where the metering block (also known as holf off valve) comes in to the braking system.

The metering block will hold off the pressurized brake fluid from going to the front disc brakes until the pressurized fluid reaches a certain pressure. This means that when the driver starts pressing the brake pedal, the brake fluid will reach the rear drum brakes and the metering block. Until the driver has pressed the pedal more to create more pressure in the system, the metering block will be closed until a certain pressure value is reached.

This prevents the brake fluid from reaching the front brakes at the same time as the drum brakes. By delaying the time, the fluid reaches the front brakes, the drum brake can overcome the return spring tension and take up the clearance between the shoes and the brake drum and start applying.

By delaying the front disc brakes, the nose dive situation is prevented because the rear drums will apply first and prevent a nose dive situation and then the front discs will also follow and create braking force after the metering block has opened. The metering block is mainly there to delay the disc brakes so that the rear drums can apply first and prevent nose dive.

They have another function which is during light braking, the metering valve will actually never see a high enough pressure in the braking system so it will never open. This means that during light braking car will only stop with the rear brakes and not with the front brakes.

This is useful because normally, during medium to heavy braking, the front brakes will do the majority of the work and wear out much faster than the rear brakes. By applying only the rears during light braking, the front brake pads are conserved and will have a longer lifetime.

The schematic above shows the internals of a metering valve. The spring inside it holds off the brake fluid from going into the front disc brakes until the fluid pressure has risen enough to win against the spring tension and open the metering valve spring so that it can flow to the front disc brakes.

If a driver slams on the brakes, during the initial pedal travel the metering block will still do its job and delay the front brakes just enough so that even if driver slammed the brake pedal and start building pressure in the brake fluid rapidly, it will not be fast enough where the metering block will not work, and the car will nose dive.

The time required to delay the front brakes is very miniscule anyway, so even if the driver smashes the brake pedal, it can’t create pressure fast enough where the front brakes will apply first. The metering block will apply the rears and with the rising fluid pressure, it will open and start applying the front brakes as well without nose diving even in emergency brake pedal slamming situations.

It should also be noted that in full drum brake cars there used to be a valve called „residual check valve”. This valve used to still keep some pressure in the brake lines which was just enough to overcome the return spring tension inside the drums so that when driver pressed the brake pedal it reduced the engagement time of the drum brakes and also reduced the brake pedal travel.

However, in most front disc rear drum brake cars, this valve was no longer used and the metering block basically replaced it. There are some exceptions, such as the 1st generation Chevrolet Camaro. On the front disc rear drum Camaros, there was still a residual check valve built into the master cylinder but only to the rear outlet of the master cylinder.

This valve kept roughly 8-16 psi of tiny fluid pressure at the rear drum brakes to overcome the return spring tension to make the rear drums engage faster. However, because there is still a gap between brake shoes and the brake drum, they still have some delay compared to disc brakes, so the metering valve was still used in this car to delay the front disc brakes.

One final note about metering blocks, they actually have 2 different springs inside them. The main spring’s function was explained above, but there is also a second, weaker spring that is connected to the valve stem. When driver is not pressing the brake pedal, the weaker spring allows a small amount of fluid movement to the front calipers and this is done to allow the fluid to expand and contract according to different temperatures.

When the driver presses the brake pedal, the weaker spring’s tension is immediately overcome by the fluid pressure due to tension being very little and this way any sort of fluid flow to the front calipers is blocked until the pressure rises enough to overcome the tension of main spring. This phenomenon can be seen in the schematic below.

The schematic below is also a good representation showing where the metering block goes in the braking system. It also shows the location of the proportioning valve which we will take a look at next.

Proportioning valve and safety systems of the 60s and 70s

1960s and 1970s braking systems are always referred as not having ABS incorporated into them except a couple of outliers such as Jensen FF from late 1960s, Chrysler Imperials from early 70s and Mercedes W116 from late 70s however 99.99% of 1960s and 1970s cars did not have ABS as we know it today where each brakes pressure is constantly adjusted to prevent wheel lockup.

However, cars back then utilizing front disc rear drum brakes still had a pseudo ABS system that prevented the rear wheels from ever locking up thanks to a device called „proportioning valve”. The rear wheels are the worst wheels to lock up because if the front wheels lock up, the car will understeer and if it crashes it will crash head on where it provides the most safety.

However, if the rear wheels lock up, the car will spin out and will crash at some random object from a random angle, which might be through the driver door and inflict the most damage on the driver. So compared to the front wheels, it was always a priority to prevent the rear wheels from locking up.

Proportioning Valves

Proportioning valves were not used in the majority of 4 wheel drum brake cars, but they became a necessity when front disc and rear drum brake cars started appearing in the 1950s. Normally, on a 4 wheel drum brake car, the total brake bias is adjusted by the size of the front and rear drum brakes.

Since most of the braking is done in the front, the front drum brakes would be much bigger than the rear drum brakes and this way a front biased brake bias can be utilized. The hydraulic system sent the exact same amount of fluid with the exact same amount of force to each drum brake, so internally the brake bias was 50/50.

However, because the front brakes were bigger, they created more braking force than the rear smaller drums, so the overall brake bias was front biased as it should be. Since both front and rear brakes were drum brakes, there was no need to apply more fluid pressure to the front brakes compared to the rear brakes thanks to drum brakes self energizing.

As stated before, disc brakes do not have self energization, so they require a lot higher hydraulic force to generate braking force in disc brakes. The required fluid force on disc brakes can be several times of that of the drum brakes to achieve the same amount of braking force.

This means that in a front disc rear drum brake system, it needs to generate a lot of fluid pressure in the braking system to be able to properly use the front disc brakes which this is generally done with the help of a brake booster servo. However, this very high amount of fluid pressure will instantly lock up the rear drum brakes because they require a lot less fluid pressure thanks to their nature with self energization.

This is the reason the proportioning valve was necessary on a disc-drum setup but not necessary on a pure drum or pure disc setup.

The proportioning valve automatically limits or reduces fluid pressure going to the rear brakes after a certain point of brake pedal travel.

There are 3 different types of valves that were used over the years that did this job. The first and the simplest one was known as a „limiting valve” and this is technically not a proportioning valve, but its aim was the same as a proportioning valve to prevent rear wheels from locking up, so it is included in this list.

Limiting Valve

The limiting valve is the simplest of them, it is a basic spring tensioned valve that when the fluid pressure inside the valve overcomes the spring tension, the valve shuts off fluid flow to the rear brakes completely so that a certain amount of pressure is maintained in rear brakes as it can be seen in the graph below. The front pressures keep rising with more pedal travel, but the rear pressure will be completely the same after the valve shuts off fluid flow to the rear brakes.

While this method works to prevent the rear wheels from locking up, it is also wasteful. The rear brakes cannot be pushed to their full potential with this method, so the total braking distance will also increase due to the rear brakes not being utilized to their full potential.

Pressure Sensitive Proportioning Valve

The second type of valve is the ” pressure sensitive proportioning valve”. The proportioning valve after a certain point starts limiting the amount of fluid pressure going to the rear wheels. It does not completely block the flow to the rear brakes like the limiting valve. It allows the rear brakes to still receive increasing pressure but at a decreased rate compared to the front brakes, which can be seen in the graph below.

The constant line is the front brake pressure, and it increases linearly as brake pedal is depressed. The other line shows the brake pressure going to the rear brakes, until the split point it receives exactly the same amount of pressure as front, but after the split point it starts to receive less pressure compared to the front brakes.

This means that rear brakes can be utilized very close to their full potential instead of being cut off short like the limiting valve did. This way, the total braking distance can be reduced while still having a safety system that prevents the rear brakes from locking up.

As it was stated before, the majority of 4 wheel drum brake cars did not have a proportioning valve and that they bias the braking force towards the front by using bigger front and smaller rear brakes. This is also true in some vehicles utilizing 4 wheel disc brakes.

Since those systems do not combine both types of brakes and hence they can use the same fluid pressure for each brake, they can bias the braking towards the front by using different size brakes. However, a proportioning valve was also sometimes used in full drum brake cars, and it also became popular in full disc brake cars in the 80s and 90s.

The method of utilizing different size brakes while working, was also less than ideal. Near the limit it performed well, it utilized the rear brakes close to their full potential and prevented them from locking up.

However, during normal braking where the driver never presses the brake pedal more than halfway, the rear wheels were doing less of a job and the front brakes were doing the majority of the job.

This can be solved by using bigger rear brakes and also a proportioning valve. This way, the rear brakes can put down more braking force when braking normally so that the lifetime of the front brakes can increase thanks to the rear brakes taking more part in braking normally, which is the scenario that happens the most with breaks because panic stops do not happen often.

When a panic stop is required, the proportioning valve will decrease the rate of pressure increase at the rear brakes so that the bigger rear brakes will not lock up in a panic stop. The proportioning valve additionally provided an even finer tuning option for the braking system, so the panic stops were also improved a bit compared to braking systems without it.

Due to this reason, a lot of 4 wheel disc brake cars also utilize proportioning valve and some 4 wheel drum brake cars in the 1960s also utilized the proportioning valve for this reason.

One example is the 1st generation Chevrolet Camaro. They came with 4 drum brakes as standard and as an option, front disc brakes can be purchased and in 1969, to homologate it in trans-am racing, a 4 wheel disc brake option was also offered.

On the full drum brake Camaros in 1967, normally a proportioning valve was not used. However, if the C60 air conditioning option was purchased, a proportioning valve was used along with it. The ac option increased the weight up front quite a bit, so to reduce front brake wear and provide an even finer tuning between front and rear brake bias, the proportioning valve was used on a 4 wheel drum brake car.

The operation of the proportioning valve can be seen very well in this video at timestamp. During light braking, the brake fluid will follow the path shown in red inside the proportioning valve body, as it can be seen below. At this stage, the fluid flows to the rear brakes without any disturbance so both front and rear brakes achieve the same amount of brake fluid, giving an internal brake bias of 50/50 between front and rear brakes.

When the driver starts to apply more force on the brake pedal, the pressure in the brake fluid increases and this pressure starts to build inside the proportioning valve on each side of the movable poppet piston as shown in red in the schematic below.

The poppet piston is held in its position by a pressure spring that surrounds the piston and also by fluid pressure shown in the red section towards the right on the small end of the piston. These two forces try to keep the piston pushed towards the left side so that its passage in the middle will not be blocked by the plunger shown in yellow. During light braking there is not enough fluid pressure to overcome the springs, so the passage isn’t blocked, and brake bias is 50/50.

However, as the driver presses harder on the brake pedal and the pressure in the fluid rises, the red section shown in the left part of the picture above starts to exert a greater force on the poppet piston that the fluid pressure on the small end of the piston and the spring force combination. This means that the poppet valve will start moving towards the right and the plunger will start blocking its passage and, at one point, will completely stop the fluid flow to the rear brakes as it can be seen in the picture below.

When the passage is completely blocked, it means that the pressure at the rear brakes is held momentarily constant, meanwhile the pressure at the front keeps rising. This is when the split point is reached, as it was shown in the proportioning valve graph from earlier.

Normally, the limiting valve mentioned earlier would keep it completely blocked and never allow more pressure to reach the rear brakes, however, as stated before, proportioning valves don’t do this, they will still keep increasing pressure to the rear brakes at a reduced rate.

This is accomplished via the next step in the operation of the proportioning valve. When the passage is completely blocked, a lot of fluid builds up on the small area of the poppet piston shown as the thick red sections in the picture below and now this force offsets the force on the big end of the piston shown as the thinner red lines on the left.

This means that now the poppet piston will start moving back towards the left section where it originally was and when this happens, the passage towards the rear brakes also opens so now fluid flow to the rear brakes continues. This can be seen in the schematic below with the green section showing the passage being opened up again.

This is the basic idea of operation behind the proportioning valve. As the driver keeps increasing the force on the brake pedal, the poppet piston will repeat this cycle left and right and this way proportion the amount of brake pressure reaching the rear brakes compared to the front brakes giving us a graph as it can be seen below.

The „crack point” in the graph below is the split point as it was talked about earlier. The poppet valve starts moving to the right when the split point is reached in the pressure curve. The split points location in the pressure curve depends on the strength of the pressure spring inside the proportioning valve.

The shape of the curve, on the other hand, is controlled by the relative sizes of the large and small ends of the poppet piston. By changing the sizes of each section, the exact shape of the curve can be altered, which means the exact amount of pressure going to the rear brakes can be altered.

Additionally, whenever this split point is reached, the brake bias within the hydraulic system is no longer 50/50. It will start to be more biased towards the front brakes and the more the driver presses the brake pedal, the more it will be front biased.

This is because more and more pressure is being exerted on the front brakes while it is being increased at a reduced rate on the rear brakes, so the brake bias between front and rear brakes change constantly and gets more and more biased towards the front as the brake pedal is pressed further.

So in front disc rear drum brake systems, the brake bias is regulated by the proportioning valve instead of the size of braking systems themselves like in most full drum brake cars. As mentioned before, non full disc brake cars it is also not necessary because the total brake bias can be adjusted by the sizes of the braking elements themselves, but many manufacturers still utilize proportioning valves in full disc brake systems since 1980s as it was explained earlier. In front wheel drive cars generally 2 proportioning valves are used which we will get to later.

Fixed Type Proportioning Valve

There are 3 different types of proportioning valves known as fixed type, screw type and lever type. OEM generally used the fixed type which, as its name indicates, was fixed and had no adjustment on it. It was calibrated for a specific car and was not adjustable.

Lever Type Proportioning Valve

The lever type is operated by a lever and can change the split point and the shape of the brake pressure curve according to drivers needs. The lever type only allows for a couple of stages of adjustment.

Screw Type Proportioning Valve

The most common aftermarket type is the screw type. The screw type allows the crack point and the curve to be changed infinitely, so it is used in racing etc. where very fine adjustment is needed. The graph below shows the graphs for lever and screw type proportioning valves. As it can be seen in the graph, the screw type is infinitely adjustable between maximum and minimum points, meanwhile the lever type has 7 predetermined positions.

Load Sensing Proportioning Valve

The last type of proportioning valve is the „load sensing” proportioning valve and this type of valve is mostly used in pickup trucks and vans where there is a drastic difference in their weight distribution when they are empty and when they are loaded.

As pickups and vans get loaded, a lot more weight is now being put on the rear axle so that rear brakes can do more job during braking, but when they are empty, the rear axle is very light. This means that a normal proportioning valve will not suit these vehicles according to their weight, which causes a significant difference when they are loaded and when they are empty.

The load sensing valve is connected to the rear suspension as it can be seen in the schematic below. As the height of the rear suspension changes, it also influences the operation of the proportioning valve. Mainly it will change the split point in the graph so that the split point can suit differing weight conditions to provide the best braking possible while at the same time always preventing the rear axle from locking up.

Recall from the earlier type of proportioning valve that there was a pressure spring inside that influenced the split point in the curve depending on the spring tension. The load sensing valve uses the same spring, however the spring is now moved to the outside and is controlled by the height of the suspension as it can be seen below.

As the vehicle is loaded and the suspension compresses, it causes this pressure spring to be stretched. This stretching of the spring causes the split point in the graph to also come later so that the rear brakes will keep receiving the same amount of pressure as the front brakes for a longer time because the vehicle will have a lot more traction at the rear wheels when it is loaded.

In the opposite case, when the vehicle is empty, the suspension is no longer compressed so the pressure spring itself now compresses which lowers the split point in the graph. The graph below shows how the split point changes according to load on the vehicle. It can infinitely change between a maximum and minimum point, much like the screw type normal proportioning valve shown earlier.

The first circle shows when the vehicle is empty and the second circle shows when the vehicle is loaded.

In the modern day, when cars have had ABS systems for decades at this point, the mechanical proportioning valve is also mostly forgotten.

Dual Circuit Braking Systems

Another safety mechanism became common in braking systems towards the late of the 1960s and this was the dual circuit braking system accomplished with the help of a dual master cylinder. This system used 2 completely separate brake circuits so that if one of the circuits gets ruptured and fails, you will not lose the entire braking system of the car and only lose that ruptured circuit, meanwhile the other circuit will not be effected.

TT Split Braking System

There are lots of different types of this configuration and the simplest one was used in most of the rear wheel drive vehicles from the 1960s and 1970s. This layout in German is known as the „TT Split” sometimes also referred as „Black and White Split”.

In this configuration one of the circuits act on the front brakes only and the other circuit acts on the rear brakes only meaning that front and rear brakes are completely separated from each other so if the front brake lines get ruptured, the rear brakes will still keep working and a schematic showing it can be seen below.

Also note that just because front and rear are on separate circuits does not mean metering and proportioning valves will not work. They will still work because the metering valve will be on the front circuit and the proportioning valve will be on the rear circuit and do their respective jobs.

This layout was used in rear drive vehicles because in a rear wheel drive vehicle the front brakes generally do around 70% of the work meanwhile the rear brakes do 30% of the work due to the front longitudinal engine followed by longitudinal transmission behind it and a differential at the rear axle still providing some weight towards the rear axle.

In the case of losing the front brakes, the 30% effort coming from rear brakes alone was deemed satisfactory in rear wheel drive vehicles since they can still somewhat use their rear brakes.

However, this is not the case for front wheel drive vehicles. In them the brake force distribution is generally 80% front, 20% rear because the engine is transversally placed on the front axle along with a transversally placed gearbox and differential, so the rear end of the car is much lighter compared to front engine rear wheel drive vehicles.

This meant that using the TT split would only provide 20% overall braking force if the front circuit malfunctioned. The 20% force was deemed as not enough, so a new solution was required for front wheel drive vehicles.

Diagonal Braking System

This solution came in the form of a diagonal braking system as it is referred in English and referred as a „X Split” in German. In the diagonal system, as the name implies, each circuit acts on one front brake and one rear brake on the opposite side, as it can be seen in the schematic below.

This layout ensured that one of the front brakes is still being utilized in case of a rupture in one of the brake circuits. The opposing rear wheel is chosen because when a single front brake is engaged, it will create a torque around the axis of the car and pull it to one side, so using the opposite rear brake applies torque in the opposite direction to the front brake, reducing the tendency to self steer.

However, as stated the front brake puts a lot more braking force than the rear brakes, so the car will still want to self steer when braking. To prevent this, majority of front wheel drive cars utilize a negative scrub radius geometry on the front suspension which provides a self correcting effect when one of the front wheels has more resistance than the another.

So cars with negative scrub radius will still keep going straight in the event of a tire blowout or if one brake circuit is lost and not car is braking with only 1 front and 1 rear brake. The negative scrub radius when combined with the diagonal braking system is extremely useful when it comes to safety.

Sometimes rear wheel drive vehicles also used the diagonal braking system such as the Morgan cars made since 1970. They also had positive scrub radius so in case if one circuit failed, they would really try to self steer when pressing the brakes.

Another thing with the diagonal braking systems is that they actually need to use 2 proportioning valves in total. Unlike the TT system where one circuit solely powers the rear brakes and can just use one proportioning valve at the rear, the X split system must use 2 proportioning valves in total.

This is because in the diagonal split system each of the circuits power one of the rear wheels, so each circuit will incorporate a proportioning valve, so these systems are generally referred as having „dual proportioning valves” because they must use two of them as it can be seen in the schematic below.

Both the TT split and the X split systems still leave a lot on the table. When a circuit fails, the braking system is still very under utilized so the braking distance is a lot higher compared to when both circuits are working properly. To reduce the braking distance in the event of a circuit loss, more advanced systems such as „LL, HT and HH Split” braking systems were developed.

LL Split Braking System

In the LL split system, one circuit controls both front wheels and one of the rear wheels, meanwhile the other circuit controls again both of the front wheels but the other rear wheel, as it can be seen in the schematic below. The first circuit shown in red controls both front wheels and the left rear wheel. The second circuit shown in black controls both front wheels and the right rear wheel.

You might be wondering how exactly each circuit can control both of the front wheels. This is done with the use of a special brake caliper. If it is a floating type caliper it must have at least 2 pistons and if it is a fixed type caliper it must have at least 4 pistons in total. Half of the pistons in the caliper will be engaged by one circuit and the other half of the pistons in the caliper will be engaged by the other circuit.

Both circuits will complete the front braking system when they work together. In the case when one circuit ruptures in the LL split system, you will lose one of the rear brakes and also lose half of the pistons at each front caliper.

This means that the brake pads will only be pushed by half the amount of pistons, so the pads will be forced on the rotor unevenly. However, this will only happen in an emergency, so applying the pads on the rotor unevenly is not a major issue. It reduces efficiency and puts a lot more wear and tear on the components, but it’s only supposed to happen during an emergency.

Half of the pistons working also means that the driver will have to push the brake pedal more than usual to achieve similar braking effort because only half the pistons are pushing the pads on the rotor so it needs twice the force to try to achieve similar braking effort compared to when both circuits are working without issues.

In the end, this system allows the majority of the front brakes to be utilized and one of the rear brakes, so overall braking performance compared to diagonal split is a lot better in an emergency. This system by using both front brakes also means that a negative scrub radius is not really necessary.

HT Split Braking System

The HT split system is extremely similar to the LL split. The only difference is that in the HT split system, one circuit always only powers the front brakes and the other circuit always powers all the brakes. This system was popular in NSU cars such as the Ro80 and the K70 which was actually sold as VW K70 and never as a NSU despite being designed completely by NSU. The HT split system can be seen in the K70 schematics below.

This system has pros and cons compared to the LL split. In the LL split, it doesn’t matter which circuit fails, the car will have the same braking performance. The front brakes will always work with half the pistons, and one of the rear brakes will always work.

In the HT split, if the circuit that only powers the front brakes fails, then it will have superior performance compared to LL split because every brake will be working. Both rear wheels will work at their full capacity and the front wheels will work with half the pistons, so the driver will still have to press the pedal more, but you get one more rear brake reducing the braking distance.

However, if the circuit that powers all the brakes fails, then only the front brakes will work because the other circuit only powers the front wheels. In this case, it will have worse braking performance than LL split because none of the rear brakes will be doing anything. Half the pistons up front will be working again, so driver still has to press the brake pedal a lot more, just like in previous cases.

The Mercedes schematic below also does a great representation of the HT split brake system. It shows TT split on the left and HT split on the right.

HH Split Braking System

The most advanced system out of them all is the HH split system. Actually, there was a 3 circuit braking system used in the 1960s Rolls-Royce models with twin calipers per rotor in the front, but we won’t get into those in this article.

In the HH split system, both circuits always power every wheel. Every caliper must have at least 2 pistons in the case of a floating caliper and 4 pistons in the case of a fixed caliper.

One circuit will power half of the pistons at each caliper and the other circuit will power the other half of the pistons. If one circuit fails in this system, you still have all 4 brakes available but with half the pistons available in the caliper. This means that again driver has to push the pedal twice as much to achieve a similar braking effort as when both circuits are functioning.

But in the end, you still have all the brakes and will achieve the shortest braking distance out of all of these systems when one circuit fails.

Distribution Block

There is another section in the braking system called the distribution block that also purely acts as a safety function. Distribution block also known as a „pressure differential valve” is a valve that moves whenever there is a loss of pressure in either of the brake circuits. Then the movement of this valve turns a light on the interior that alerts the driver there is a loss of pressure in the braking system so that driver can be careful and immediately get it repaired.

As it can be seen in the schematic below, the distribution block has a piston that sits on a small shaft. This shaft sees pressure from both braking circuits at each end, so normally the shaft is centered on the piston during normal operation. However, if there is loss of pressure in either circuit, the shaft will move towards the section with lower pressure. Since the shaft is raised, the shaft moving left/right will also move the piston upwards and this motion of the piston will trigger a switch that will light up the brake emergency light in the dashboard. Not every car from the 1960s and 1970s had this feature, but a lot of them did have it.

Combination Valve

One final improvement to these old analog braking systems came in the late 1960s, and it was an improvement in servicing cost, and packaging rather than anything else. In the USA GM started putting a combination valve in 1971 on all of their cars.

The combination valve, as its name implies combines the functions of the metering block, proportioning valve and the distribution block all into one single small unit as it can be seen in the schematic below.

The combination valve is usually located just after the master cylinder as it can be seen below from Ford Taunus TC2.

Dangers of retrofitting disc brakes onto classic cars with drum brakes

Many people are under the impression that they need to immediately install aftermarket disc brakes on a classic car to make it daily drivable in the 21st century. This is completely wrong, and many people also do not exactly know how these systems work, so they do half jobs in these conversions without taking everything into consideration and in the end create an even worse more dangerous braking system when converting their drum brakes to disc brakes.

When doing a front disc swap while keeping the same drum brakes at the rear, many people just install the front disc brakes and not worry about anything else. They don’t change the master cylinder, they don’t put a metering block, they don’t put a proportioning valve, they don’t check if the brake pedal ratio should be the same when using front disc brakes etc.

The consequences of this is that they are creating an even more dangerous braking system than the factory full drum system. Disc brakes, as discussed earlier, require a lot more force to actuate, so their master cylinders also have bigger bores to let more fluid to flow into them and if you use the master cylinder from drum brakes, you will under utilize your disc brakes.

Disc brakes also need more overall fluid in their master cylinders because as the seals readjust the position of the piston they require more fluid overall than drum brakes.

If you do not put a metering block, your front disc brakes will engage sooner than the rear drums, and you will overuse the front brakes meanwhile rear brakes won’t contribute too much in addition to the car always diving car when braking due to lack of metering block.

If you do not put a correctly calibrated proportioning valve, in a panic stop you will immediately lock up the rear brakes and if this happened during a corner you will spin out immediately. All of these things are very dangerous to everybody on the road and must be taken very seriously.

Don’t just install custom parts on your braking system without knowing how all of them operate together. Even full drum brake cars will function absolutely fine in daily use if you keep on the maintenance, just don’t do back to back high speed stops with them to not experience fade.

Conclusion

It was a pretty deep dive about the old braking systems in the era of transition from full drums to front disc and rear drum brakes and this brought many challenges with it. If you want to retrofit your old car with a new braking system, go ahead and do it but make sure to do it correctly by paying careful attention to each part of the system because safety is very important, not only for you but also for other people on the road.