The brake systems job is to turn rotational motion into heat energy, slowing/stopping the vehicle, and then disperse that heat. The front brakes do most of the actual braking because the weight of the vehicle leans forward when the vehicle slows down. The reat brakes do provide some stopping power but are mostly used for stability during braking. There are two main types of braking systems commonly in use today; drum brakes and disc brakes. A vehicle will have one disc or drum on each wheel. The front may have discs and the rear may have drums, or all four wheels may be disc brakes. Only some classic vehicles will have 4 wheel drums. The base braking system uses hydraulic pressure to activate each wheel’s braking system.
A disc brake system uses a stationary caliper which squeezes brake pads against the rotor which rotates with the wheel. This creates friction between the rotor and the pads, this slows the rotor down and creates heat. The rotor is attached to its corners wheel through the studs and lugnuts or wheel bolts so when the rotor is slowed down, so does its wheel.
The brake caliper uses the hydraulic pressure of the braking system to apply pressure to the brake pads. Most calipers A caliper will have at least one piston but many will have multiple pistons. Calipers with one piston are floating calipers, which use the piston to apply the inner pad and the caliper itself will slide to apply the outer pad. Calipers with multiple pistons are fixed calipers and will have an equal number of pistons to apply each pad. As the pads wear down, the pistons will rest and operate farther out of their bore. When installing new brake pads, the caliper piston will need to be pressed back into its bore. Many systems require a special tool or procedure to accomplish this.
The pads job is to convert the rotational motion of the rotor, into heat energy. The pads are a friction material bonded or riveted to a steel backing. When the caliper applies the pads, the pads squeeze the rotor, which causes friction between the two. Any time the pads are applied, the pads are getting worn down slightly. When the friction material is completely worn down, the steel backing will come into contact with the rotor. This makes an ugly sound when the brakes are applied and may not provide enough friction for proper braking. Most modern vehicles will have a way to warn the driver long before this happens. Many pads have what is known as a screamer which makes a squealing noise when the brakes are applied when the pads are getting thin. Other systems will have a sensor in the pads. This sensor is simply a wire that when interrupted, when the pads wear down, illuminates a MIL in the instrument cluster. The pads work their way into the rotor over time and should be changed with the rotor.
The rotor is the braking systems input to the wheel. It is a cast iron disc which turns with the wheel. Rotors can be solid cast iron, or vented depending on the application. Vented rotors allow air to pass through the rotor for cooling as the wheel and rotor spin. If a rotor overheats the metal surface can warp. This causes the brake pedal to pulsate when the operator applies the brakes. Once this happens the rotors and pads will need to be replaced to fix this problem. Many shops will still machine used rotors on a brake lathe, but usually the cost of new rotors is less than that of the time it would take a mechanic to machine the old rotors. Many machined rotors will also tend to warp again even after machining, depending on how excessively the rotor was overheated.
In a drum brake application, two brake shoes are pushed outward by a wheel cylinder when the operator applies the brakes. The shoes friction surfaces push against the friction surface along the inside of the drum. This creates friction between the shoes and and the drum, this slows the drum down and creates heat. The drum is attached to its corners wheel through the studs and lugnuts or wheel bolts so when the rotor is slowed down, so does its wheel.
The wheel cylinder is an aluminium housing with a small piston coming out of both sides. It uses the hydraulic pressure from the braking system to push the pistons outward to push the brake shoes out into the drum. Each piston will have a seal on them to prevent brake fluid from leaking out. Also, between the pistons is a spring that keeps a slight pressure on the pistons to hold them in place.
The brake shoes job is to convert the rotational motion on the drum into heat energy. The brake shoes are a curved friction material on a steel backing. There are two shoes per drum; a primary shoe and a secondary shoe. The primary shoe is the shoe located towards the front of the vehicle, and the shoe towards the rear of the vehicle is the secondary shoe. They are curved to the shape of the drum to maximise contact surface area. When the brakes are applied, the wheel cylinders push the shoes into the drum which causes friction between the two. This slows the drum and creates heat. Shoes wear down because of this process, just like brake pads in a disc brake setup.
The drum acts as the braking systems way of controlling the wheel, similar to a rotor in a disc brake system. They are usually made out of cast iron. Its job is to provide a surface for the shoes to press on to slow/stop the drum and wheel. Because of the enclosed area created by the drum, drums do not dissipate heat as well as disc brake systems. Just like rotors, drums are also susceptible to warping and causing a brake pulsation. They will need to be replaced, along with the shoes to fully repair the problem. Machining a drum is possible but warped drums tend to warp again.
Drum brake systems can be harder to service than disc brake systems due to the extra necessary hardware. Brake shoes need to be held out of the way of the rotating drum when the brakes are not applied. Strong springs are used to accomplish this. Many different designs are used to accomplish this and can be tricky to get back together once apart. Drum brakes also need to be adjusted as the shoes wear down. Again, many different brake adjuster designs are used for this task. The backing plate is the foundation of the drum brake system that all the hardware is attached to.
Drum Brakes vs Disc Brakes
Disc brakes are considered to be the better of the two, but there is nothing wrong with having rear drum brakes. Disc brakes are used in the front of modern vehicles because the front brakes do most of the braking for the vehicle. This is because the vehicle leans forward under braking conditions. This means that much more rotational motion is being converted into heat in the front of the vehicle than in the rear of the vehicle. Discs can dissipate this heat much better than drums because of the openness of disc brakes and the enclosed nature of drum brakes. Disks get much more airflow on their friction surfaces which results in much better cooling or dissipation of heat. Drum brakes are however cheaper to produce with a cable parking brake. This is why they are still used in the rear of some modern automobiles.
Hydraulic Brake System
The hydraulic braking system converts the drivers brake pedal input into hydraulic pressure and delivers it to the calipers and/or wheel cylinders. The brake pedal is attached to a push rod which pushes the primary and secondary pistons inside the master cylinder when the driver pushes the brake pedal. The drivers leg force alone may not be enough for the braking system to stop the vehicle in all applications. This force is amplified by the brake booster which will be explained in the power brakes section. The master cylinder’s pistons push brake fluid into the brake lines which transmit the pistons force to the calipers and/or wheel cylinders.
The Master Cylinder
The master cylinder converts the operators pedal input to hydraulic force. Most have two pistons, a primary and a secondary piston. These pistons have a special seal called a cup on them. They seal when the pistons apply the brakes but allow fluid to flow past the seal when the brakes are released as the pistons return to their normal position. This allows the brake pedal and pistons to return quickly and also allow the brakes to “pump up” when the brakes are pressed several times very quickly. The primary piston is the one closest to the driver and the secondary piston is the one closest to the closed end of the master cylinder. Both will push brake fluid into their separate circuits when the brake pedal is pressed. Two separate pistons are used so that if there is a fluid leak where hydraulic pressure can escape on one circuit, the other circuit will continue to work properly. The circuits will be divided so that each piston will control the left front and right rear wheel or the right front and left rear wheel. This way if one circuit fails, the braking system still be able to stop one front and one rear wheel as well as one left and one right wheel. This ensures that even if the system has a hydraulic pressure leak, the driver can still make relatively controlled stops. The brake fluid reservoir sits on top of the master cylinder and supplies the master cylinder with fluid through ports. Each piston has two ports, a compensating port and a fill port. The compensating port sits just in front of the piston’s cup and allows fluid to flow between the reservoir and the area in front of the piston when the brakes are not applied. This allows the fluid to expand when the fluid starts to heat up without unintentionally applying the brakes. When the operator presses the brake pedal and the pistons move, they cover up the compensating ports so the only place for fluid to go is into the brake lines. The fill port is behind the pistons and allow fluid to flow from the reservoir into the area behind the piston when it moves. This fills the negative pressure area or “void” behind the piston when the pistons move.
Brake fluid is the hydraulic fluid used to transfer pressure from the master cylinder to the calipers and/or wheel cylinders when the brakes are applied. It must not compress and transfer pressure undiminished, in all directions through the system. It must be able to withstand temperatures below freezing and withstand temperatures up to its DOT boiling point. The boiling point is the temperature that the fluid will evaporate in the brake line and become a compressible vapour. High performance and heavy duty applications require a higher boiling point. Brake fluid must be able to absorb water or moisture that it is exposed to. The absorption of moisture lowers the fluids boiling point. It must maintain its viscosity through all temperatures, from below freezing to boiling point. This is important for proper lubrication of the entire braking system, especially the ABS valves and pump. It must not damage the rubber components of the braking system, as plain mineral oil would. It also must resist corrosion in the brake lines. Brake fluid slowly absorbs moisture from the atmosphere over time or may become contaminated with dirt and/or debris and will need to be replaced. Only top up brake fluid with the factory recomeded DOT rating.
Brake Fluid DOT Rating
|Dry Boiling Point||Wet Boiling Point|
|DOT 3||205°C (401°F)||140°C (284°F)|
|DOT 4||230°C (446°F)||155 °C (311 °F)|
|DOT 5.1||270°C (518°F)||190°C (374°F)|
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Brake lines transfer brake fluid pressure and flow from the master cylinder to the calipers and/or wheel cylinders. Typical brake lines are made out of rolled steel although some are made from copper or copper along with another metal. Many people have different opinions on the use of copper brake lines. All I will say about them is to beware that they are illegal in some areas. These lines must be able to hold 2000+PSI during a hard stop. Many lines are made seamless so that they can hold more pressure. This type of line is mainly used in high performance applications. Note: I do not recommend that anyone other than a licensed technician perform any kind of repair to the braking system, especially a brake line.
A flex hose is a flexible portion of the brake line circuit. It allows the suspension system to move while maintaining a connection to the caliper and/or wheel cylinder. Most factory systems use a rubber flex hose with a thin steel braid inside for support. This type of flex hose can swell up as the brakes are applied, which absorbs some of the brake lines pressure. High performance flex hoses will be made out of a flexible steel tube wrapped in steel braiding. This prevents swelling of the flex hoses and ensures that the calipers and wheel cylinders receive the same pressure that the master cylinder produced.
Brake Fluid Valves and Switches
Pressure Differential Switch
The pressure differential switch alerts the driver of a problem in one of the hydraulic circuits. This switch sits between the two main circuits. The two main circuits should always have the same pressure. If one circuit has a higher pressure than the other, that indicates a problem in the hydraulic circuit with the lower pressure. This will cause a rod to get pushed over to the side with the lower pressure and complete the brake light circuit, illuminating the light.
This valve is used on vehicles with front disc brakes and rear drum brakes. It holds off the front disc brakes until the rear drums are slightly engaged. Since the rear brakes provide stability during braking, applying them first makes for a much more stable stop. Also, brake shoes are held off the drums by springs while brake pads are in constant contact with the rotor. If the metering valve was not in place the brakes may be far to bias to the front for stable braking.
The rear brakes can lock up at a lower brake fluid pressure than the front brakes because of the self-energising action of the rear drum brakes and also because of the vehicles weight leaning forward and off the rear wheels during braking. The proportioning valve or “prop valve” allows pressure to the rear brakes up to a specified level. Beyond this the prop valve blocks pressure to the rear brakes to keep them from locking up.
Bleeding the Brakes
If the hydraulic system is opened for any reason, the system will have to be bled to resume normal brake function. When the hydraulic system is exposed to air, air enters the system. Since air can compress, this will cause a “spongey” brake pedal and a loss in brake line pressure. The air must be purged from the system for the brakes to work properly.
The power brake system amplifies the drivers leg pressure to the master cylinder to increase brake line pressure with less driver effort. Even without power brakes, the driver has some mechanical advantage over the push rod to the master cylinder from the lever action of the pedal. This may not be enough for all braking applications. There are two main types of power brake systems; vacuum power brakes and hydraulic power brakes.
Vacuum Power Brakes
Vacuum power brakes use vacuum and atmospheric pressure to amplify the drivers leg force to the master cylinder. Most systems will use intake manifold vacuum as a vacuum source. They will simply run a hose from the manifold to the brake booster. Diesel engines do not have manifold vacuum and will need either a mechanical pump driven by the engine or an electric pump. Some gasoline engines will use an electric pump along with manifold vacuum and some will only use an electric pump. No matter how the system creates vacuum or a low pressure area, it will use it in the same way.
The brake booster is a large (usually) black, circular container that sits between the firewall and the master cylinder. The booster is divided into two halves by a diaphragm which is connected to the brake push rod. The half that faces the engine is supplied vacuum at all times by the vacuum hose which may have a one-way valve on it to avoid pressure entering the booster if the engine backfires. The half that faces the firewall has two ports which control its pressure, the vacuum port and the atmospheric port. The vacuum port connects the front and rear halves to apply vacuum to the rear half. The atmospheric port connects the rear half to the air inside the cabin near the drivers feet through a small filter. When the driver is not pressing the brake pedal, the vacuum port is open and the atmospheric port is closed, so vacuum is present on both halves of the booster. When the driver pushes the brake pedal, the vacuum port is closed and the atmospheric port is opened. This causes air to flow into the rear half of the booster through the small filter until atmospheric pressure is present. Since vacuum is present in the front half and atmospheric pressure is present in the rear half, this causes the atmospheric pressure to press on the diaphragm that separates the two halves. It is this pressure that assists the drivers foot in creating pressure for the master cylinder since the diaphragm is connected to the brake pedal push rod. When the driver holds the brake pedal in one place, the atmospheric port closes and brake assist remains the same. When the brake pedal is released, the atmospheric port remains closed and the vacuum port is opened. This causes the air in the rear half to be pulled into the engine until vacuum/pressure is equal in both halves. It is worth noting, that this small amount of extra air can cause and engine to run lean for a moment and the PCM may have to adjust for this unmetered air to avoid the engine stumbling when the brakes are released.
Hydraulic Power Brakes
Hydraulic Power Brakes use the hydraulic pressure of the power steering system to amplify the drivers leg force to the master cylinder. These systems are actually very simple but are commonly misunderstood. The hydraulic assist unit sits between the firewall and the master cylinder and consists of four main parts: the brake pedal input rod, the lever assembly, the spool/control valve and the apply piston. The control valve controls power steering fluid flow into the hydraulic brake assist unit. When the brakes are not applied, all of the power steering fluid is directed to the steering rack or gear. When the driver pushes the brake pedal, the spool valve is moved to a position which allows pressurised power steering fluid to enter the housing. The harder the pedal is pushed, the more fluid is allowed to enter the housing. This pressurised fluid pushes on the apply piston which is connected to the master cylinder. It is the fluid pushing on the apply piston that provides brake assist for the driver. When the brake pedal is released, the spool valve is returned to its original position, which allows the pressurised fluid to return to the power steering reservoir and the apply piston to return to the off position. The system also keeps a backup supply of pressurised power steering fluid which is good for one time use if the hydraulic system fails. This should be just enough for one controlled stop to the side of the road. It is important to note that if the hydraulic system does fail, and the backup supply is depleted or does not function, the driver is at a mechanical disadvantage over the master cylinder because of the lever action of the hydraulic unit. The vehicle may be near impossible to stop with the base brakes and the emergency brake will be your best bet in this condition.
ABS brakes have been the at the centre of controversy in the automotive world since they were first introduced. ABS stands for Anti-lock Braking System. Its main purpose is to prevent any of the wheels from locking up during hard braking to avoid skidding. Braking to the point that the wheels are no longer rotating with the vehicle still moving results in skidding. When this happens, the driver is no longer in control of the vehicle. The purpose of ABS is to avoid wheel lock-up/skidding while braking so the driver can maintain control of the vehicle in a panic stop. ABS was not designed to make the vehicle stop faster, it was designed to allow the driver to steer the vehicle in an emergency braking situation. Although, ABS will allow a vehicle to stop faster than than a non-ABS equipped vehicle under normal driving conditions (dry pavement) by maintaining traction on all 4 wheels during hard braking. However! ABS will increase stopping distance if engaged on gravel or in deep snow. But remember, it was not designed for shorter stopping distances anyway. When the ABS does engage, it is important not to panic. The pusing brake pedal and groaning noise is normal. Understand that pushing harder on the brake pedal will not make the vehicle slow down any faster once tha ABS has engaged. Focus on steering the vehicle to a safe stop, which is what the ABS system was designed to allow you to do in the first place.
The ABS system has sensors on (usually) all four wheels which detect individual wheel speed. This information is sent to the ABS module which is the brain of the ABS system. The system continually monitors the speed of all four wheels. When the brakes are applied, the module monitors the rate at which the wheels slow down. If the system determines that one or several wheels are slowing down faster than the others or all four wheels are slowing down too quickly, the ABS must activate. The ABS wants to accelerate the wheels that are slowing down too quickly. To do this, the system uses two valves per wheel, the isolation valve and the dump valve. The isolation valve is normally open to allow fluid flow, and the dump valve is normally closed to block fluid flow. The first step is to cut off more pressure from the master cylinder. To do this the module closes the isolation valve. This causes the pressure in the line to remain constant, even if the driver pushes harder on the pedal. If the wheel(s) do not accelerate back up to the modules desired speed, the module will reduce brake fluid pressure by pulsing open the dump valve which allows fluid to flow into an accumulator. Pressure is bled off until the wheel(s) accelerate to a speed acceptable to the module for controlled braking. At this point, the module will stop pulsing the dump valve and reopen the isolation valve and the driver is back in control of the brake fluid pressure to that wheel(s). The system will also activate the ABS pump, which will pump fluid from the accumulator back into the circuit. This entire process can happen on any one or all of the wheels brake fluid circuits up to 30 times per second.
The brake switch informs the ABS module when the brakes are in use. Some newer systems may also detect pedal pressure and/or speed to determine if the driver is panic braking.
ABS Wheel Speed Sensors
Wheel speed sensors are usually a PM generator type of sensor. They ride on a reluctor wheel either on the spindle or on the axle. They inform the ABS module of wheel speed with an AC frequency signal. These sensors are common to fail because they are constantly exposed to elements and also the vibrations of the suspension system.
The module is the brain of the system, it receives inputs from the wheel speed sensors and the brake switch to determine when to engage the ABS. When the system does decides to engage, the module activates its internal solenoids which directly controls the hydraulic units valves. The systems settings and/or sensitivity are programmed from the factory for the application. Some of this information may be VIN specific. The module is usually attached to the hydraulic unit.
ABS Hydraulic Unit
The hydraulic unit houses the isolation valve, the dump valve, the pump and the accumulator(s). This is where the actual ABS function happens. It is controlled by the module which may be a separate piece or an integral piece with the hydraulic unit. If this piece gets air into its circuits, it may be necessary to use a scan tool to bleed the unit with the ABS pump to get the air out.
Types of ABS Systems
This early type of ABS was designed to prevent rear wheel lock-up only. They will use a wheel speed sensor on each rear wheel and a single brake line to the rear which splits to each rear wheel. During braking, most of the vehicles weight is leaning to the front of the vehicle and off the rear wheels. This increases the chance of the rear wheels locking-up and causing a skid. Since only one brake line goes to the rear of the vehicle, the ABS can only pulse both rear brakes together.
This type of system has a wheel speed sensor on all four wheels, it has a separate brake line to each front wheel but still only one brake line going to the rear wheels. This system can pulse the front brakes separately, but has to pulse both rear brakes together, even if only one wheel locks-up.
4 channel systems have a wheel speed sensor on each of the four wheels, and a separate brake line to each wheel as well. This system can pulse any of the wheels independently to each other if necessary. This is the best of the ABS systems, but also the most complex.