Brakes: Braking Systems

Braking fundamentals	Principles of braking	Friction between braking surfaces converts kinetic energy into heat. In drum brakes, wheel cylinders force brake linings against the inside of the drum. In disc brakes, pads are forced against a brake disc.
	Drum & disc brakes	Drum brakes have a brake shoe that expands against the inside of a drum. Disc brakes clamp a flat disc between two pads. An antilock braking system is a control system that applies maximum braking force without wheel-lock or skidding.
	Coefficient of friction	The coefficient of friction is the measurement of friction between pairs of surfaces.
	Lever/mechanical advantage	A lever allows small efforts to overcome large loads – or vice versa, depending on the type of lever. The ratio between load and effort for any given lever is known as the Mechanical Advantage.
	Hydraulic pressure & force	Hydraulic pressure is transmitted through liquid & can transmit increased force.
	Brake fade	Brake fade is the reduction in stopping power caused by a buildup of heat in the braking surfaces.
	Regenerative braking	Regenerative braking is any technology that allows a vehicle to recapture and store part of the kinetic energy that would ordinarily be lost when braking.

Braking systems	Brake type – principles	Drum brakes have a drum attached to the wheel hub, with disc brakes a disc attached to the wheel hub maybe clamped between two brake pads, and ABS prevents the wheels from locking while braking.
	Air brakes	Air brakes are used on heavy vehicles and use compressed air to provide the large braking forces required.
	Exhaust brakes	An exhaust brake works by restricting the flow of exhaust gases through the engine, which slows engine rotation.
	Electric brakes	Electric brakes are commonly used with trailers and are activated when the brakes in the towing vehicle are applied.
	Parking brakes	Most light vehicles use a foot brake that operates through a hydraulic system on all wheels, and a hand-operated park brake that acts mechanically on the rear wheels only.
	Engine brakes	Engine braking, or compression braking, uses the resistance of the engine during the compression stroke to slow the momentum of a vehicle. A 'jake' brake on larger vehicles controls a second exhaust valve on the engine to maximize the braking effect of the engine.

Braking system components	Park brake system	The primary components of the park brake system are: parking brake cables, and disc brake caliper fitted with park brake, or park brake lever on drum brakes.
	Brake pedal	The brake pedal acts as a lever to increase the force applied to the brake pedal pad, by the driver.
	Brake lines	Brake lines carry brake fluid from the master cylinder to the brakes. For most of their length they are steel, and attached to the body with clips or brackets to prevent damage from vibration. They are much the same on all brake systems.
	Brake fluid	Brake fluid is a special purpose high-boiling point fluid. It transmits the hydraulic pressure generated by the master cylinder to the brake units.
	Bleeding	When you bleed a hydraulic system you are removing air from the system.
	Master cylinder	The single-piston master cylinder transforms the applied pedal force into an hydraulic pressure which is transmitted simultaneously to all four wheels.
	Divided systems	Braking ability depends on the load on a wheel during braking. Front-engined, rear-wheel cars use a braking system with a front-rear split, or with an L-split. Most front-engined, front-wheel drive vehicles use a diagonal configuration.
	Tandem master cylinder	The tandem master cylinder transforms applied brake force into hydraulic pressure which is transferred to the wheel units through two separate circuits. This provides residual braking in the event of fluid loss.
	Power booster or brake unit	The power booster assists the driver by reducing the amount of effort that has to be applied to the brake pedal during braking.
	Hydraulic brake booster	The hydraulic brake booster uses hydraulic pressure generated by the power steering pump rather than engine vacuum to provide the power assistance required.
	Electrohydraulic braking (EHB)	Electrohydraulic braking replaces the current modulator with one that includes a high pressure accumulator.
	Applying brakes	As the brakes are applied, the pedal pushrod transmits movement through the power unit to the master cylinder piston to apply the brakes.
	Brake force	The power booster uses pressure differences in a diaphragm unit to increase the force being applied in the brake circuit.
	Brake light switch	The function of the stop or brake light switch is to indicate to people driving behind us that our vehicle is slowing or stopping.

Drum brakes & components	Drum brake system	The primary components of the drum brake system are: the backing plate, the brake drum & brake shoe assembly, the wheel cylinder, retaining clips & springs and automatic brake self-adjuster.
	Drum brake operation	The drum brake system operates by forcing the friction-lined brake shoes against the inner surfaces of the rotating drums. The shoes are designed to operate with a self-energizing action.
	Brake linings & shoes	The brake lining is a specialized friction material, riveted or bonded to a steel shoe. The shoe transfers the wheel cylinder movement and forces the lining against the rotating drum.
	Backing plate	The backing plate is bolted to the axle housing or suspension member. It provides a mounting for the wheel cylinders and brake shoes to act against the rotating drum.
	Wheel cylinders	The wheel cylinder and piston react to hydraulic pressure from the master cylinder, and the outward movement of the piston forces the shoe and lining against the drum.

Disc brakes & components	Disc brake system	The primary components of disc brakes are: the rotor, caliper and brake pads.
	Disc brake operation	The disc brake system converts the hydraulic pressure generated at the master cylinder into a frictional clamping force against the rotating discs.
	Disc brake rotors	The brake disc rotates with the road wheel. It provides a smooth surface against which to force the brake pads, to slow or stop the vehicle.
	Disc brake pads	A disc brake pad has a rigid, molded, friction material bonded to a steel backing plate for support during brake application. It transforms the hydraulic force of the caliper into a frictional force against the disc.
	Disc brake calipers	Disc brake calipers provide a housing for the hydraulic piston or pistons that force the brake pads into contact with the disc.
	Proportioning valves	The proportioning valve divides up the braking effort applied to front and rear wheels under heavy braking, according to how load is distributed across a vehicle.
	Proportioning valve operation	The proportioning valve adjusts the braking force applied to the wheels to allow for changes in load.
	Brake friction materials	Brake pads and linings are made from materials which have a high coefficient of friction. The choice of material depends on the braking application, but it needs to be able to absorb and disperse large amounts of heat without braking performance being adversely affected.

Antilock braking system & components	ABS brake system	The primary components of the antilock braking system are: The electronic control unit, hydraulic control unit or modulator, power booster & master cylinder assembly and wheel sensor unit.
	Antilock braking system operation	The antilock braking system prevents wheels locking or skidding, no matter how hard brakes are applied, or how slippery the road surface. Steering stays under control and stopping distances are generally reduced.
	Principles of ABS braking	The antilock braking system controls braking force by controlling the hydraulic pressure of the braking system, so that the wheels do not lock during braking.
	ABS master cylinder	The tandem master cylinder transforms applied brake force into hydraulic pressure which is transferred to the wheel units through two separate circuits. This provides residual braking in the event of fluid loss.
	Hydraulic control unit	The hydraulic control unit, or modulator, executes commands in the form of electrical signals from the ABS Control Module. It uses solenoid valves to change the hydraulic pressure in the brake circuit.
	Wheel speed sensors	Wheel speed sensors consist of a toothed rotor and a pickup. Wheel rotation sends input signals to the ECU, which processes them and controls the hydraulic control unit.
	ABS electronic control unit	The ECU receives signals from various sources. The brake pedal, the ignition system, and wheel speed sensors control the hydraulic control unit and anticipate wheel lock.

Brakes procedures	Checking & adjusting brake fluid	The objective of this procedure is to show you how to check and adjust brake fluid level. Brake fluid will absorb moisture rapidly. Always replace the cover or lid as soon as possible.
	Replacing brake fluid	Most manufacturers recommend that brake fluid be changed at least every two years. The objective of this procedure is to show you how to test and replace brake fluid in accordance with manufacturer's recommendations.
	Checking brake pads	You should check the disc brakes and disc brake linings every 15,000 kilometers (10,000 miles). The objective of this procedure is to show you how to check brake pads and calipers as well as check and measure rotors.
	Replacing brake pads	The objective of this procedure is to show you how to replace brake pads and refit according to the manufacturer's recommended procedure. Remember to remove some of the brake fluid from the master cylinder before you raise the vehicle.
	Removing & replacing a rotor	Before working on a vehicle's brakes, attach a safety notice to the vehicle to prevent people attempting to operate the brakes. The objective of this procedure is to show you how to safely remove and reinstall a disc brake rotor.
	Checking wheel cylinders	Some vehicles still have drum brakes or a combination of discs and drums. Passenger cars with this combination will have the drum brakes on the rear. Brake linings should be checked approximately every 15,000 kilometers (10,000 miles). The objective of this procedure is to show you how to check wheel cylinders, drum brake linings and measure brake drums.
	Replacing brake linings	Linings are designed to withstand extreme heat generation during braking. The linings must be able to withstand these temperatures and still maintain their braking efficiency. The objective of this procedure is to show you how to remove and replace drum brake linings.
	Adjusting a park brake cable	There are two main ways of adjusting park brakes on vehicles; one method is done from underneath the vehicle, and the other from inside. A vehicle will only be fitted with one method of adjustment.The objective of this procedure is to show you how to check and adjust park brake cable following the manufacturer's procedure.

Topic group	Topic	Summary