Disc brakes are a type of brake system that uses flat rotors or discs as the friction surface for a pair of brake pads. The pads clamp against both sides of the rotors to apply the brakes. Most cars and light trucks since the early 1970s have front disc brakes, and many have rear disc brakes also. Disc brakes provide more stopping power, better cooling and resist brake fade much better than drums if the brakes get wet.
Alternatively, drum brakes are a type of brake system that uses a cylindrical drum as the friction surface for a pair of brake shoes. The shoes push outward against the inside of the drum to apply the brakes. Many vehicles still have drum brakes for the rear wheels.
A pair of pistons inside the master cylinder create hydraulic pressure when the brake pedal is depressed. This pushes fluid through the brake lines to each wheel to apply the brakes. Fluid is stored in a reservoir atop the master cylinder. The master cylinder has two separate hydraulic circuits. Each circuit operates two of the four brakes (both fronts, both rears or a diagonal pair). This is a safety requirement so that if one side fails, at least two brakes will continue to operate.
Wear in the master cylinder may allow fluid to leak past the piston or shaft seals. A symptom of a bad master cylinder is a brake pedal that slowly sinks to the floor when braking at a stop light. Leaks or failure to hold pressure require rebuilding or replacing the master cylinder. Rebuilding aluminum master cylinders is not recommended. On some older vehicles with antilock brakes (ABS), the master cylinder may be part of the ABS modulator and is very expensive to replace.
PRESSURE DIFFERENTIAL VALVE
The pressure differential valve is a safety switch that turns on the brake warning light if there's a loss of pressure or fluid in either hydraulic circuit.
The proportioning valve reduces hydraulic pressure to the rear brakes to maintain proper brake balance. It's located on or near the master cylinder. Some vehicles have a "load sensing" proportioning valve attached to the rear suspension to vary brake pressure according to vehicle load.
A glycol based hydraulic fluid is a fluid used in brake systems to apply the brakes. Two basic types are used: DOT 3 and DOT 4. DOT 4 has a higher temperature rating than DOT 3. Use the type specified by the vehicle manufacturer. Brake fluid absorbs moisture and becomes contaminated over time. This promotes corrosion inside the brake system and lowers the fluid's boiling temperature, which increases the risk of pedal fade and brake failure during hard use. Many experts recommend replacing the brake fluid when the brakes are relined or other major brake repairs are performed. Some type of bleeding equipment (pressure or vacuum) may be needed to get the air out of the lines when brake lines are opened or the fluid is changed.
Calipers squeeze the brake pads against the rotors in a disc brake system. Most calipers have one or two pistons, but some have up to four. Most calipers are cast iron (though some are aluminum) and have steel or molded phenolic (plastic) pistons. Most calipers are a "floating" design with slides or bushings that allow the caliper to move sideways and center itself over the rotor when the brakes are applied. There are also "fixed" calipers that do not slide.
On vehicles with four wheel disc brakes, the rear calipers may also include some type of parking brake mechanism. This makes the calipers more complicated and expensive to replace.
Common problems with calipers include fluid leaks due to worn piston seals and sticking which may be due to corrosion around the piston or corroded slides or bushings. Uneven pad wear (inner pad worn more than the outer pad) is a common symptom of caliper sticking. Leaky, sticking or damaged calipers should be rebuilt or replaced. Many brake experts also recommend rebuilding or replacing the calipers on high-mileage vehicles when the brakes are relined to prevent problems later.
Replacement calipers should have the same type of piston (steel or phenolic) as the original if only one side is being replaced. Mounting hardware such as pins, bushings and slides should also be replaced when changing calipers and should be lubricated with a high-temperature moly brake grease. "Loaded" calipers come with complete pads and hardware for easy installation.
Disc brake rotors may be solid or vented (cooling fins between the faces). One cannot be substituted for the other. On some vehicles, the rotors are directional (left and right are different). Different types of rotor construction is also used. Some rotors are made of cast iron, while others use a "composite" design with a stamped steel center hat and cast iron disc. Composite rotors can be more troublesome because they flex more than solid rotors, and they can be more expensive to replace. So cast iron replacement rotors are available for many vehicles that were originally equipped with composite rotors. Some brake experts don't think this is a good idea (substituting one type of rotor design for another) because it can create changes in wheel geometry and braking action.
Rotors must be replaced when worn to minimum service or discard specifications. New rotors are also needed if the rotors are cracked or have hard spots. Resurfacing used rotors can restore flatness and parallelism and is recommended when new pads are installed. On vehicles with "captured" rotors located behind the hub, replacement can be difficult and may also require replacing the wheel bearings.
The hydraulic component inside a drum brake is the wheel cylinder. It pushes the shoes out against the drum. The wheel cylinder has two opposing pistons that move outward when pressure is applied. The wheel cylinder is mounted on the brake backing plate, and it has dust seals over the pistons to keep out dust and water. Each piston has a cup-shaped seal for the fluid inside.
Common problems with wheel cylinders include fluid leaks and sticking. Wheel cylinders can be rebuilt or replaced. Leaking fluid can contaminate the brake shoes requiring their replacement as well.
Drums are usually made of cast iron. The drum provides a friction surface for the shoes to rub against. As the drums wear, they may become grooved, out-of-round, develop a bell-mouth shape or crack. If the drum is worn beyond maximum diameter or discard specifications, is cracked or has hard spots, replacement is necessary. Like rotors, drums should be resurfaced when new brake shoes are installed. Removing drums can be difficult due to rust and corrosion, so a drum puller may be needed to pull the drum loose.
Drum hardware consists of: the return springs, holddown springs, self-adjusters and other cables, clips or springs used in the brake assembly. Return springs that pull the shoes back away from the drum when the brakes are released may become weak with age, allowing the brakes to drag. Self-adjusters can become corroded and stick, causing increased pedal travel as the shoes wear. Recommend new hardware when the brakes are relined.
BRAKE HOSES AND LINES
Carrying hydraulic pressure from the master cylinder to each brake are the brake hoses and lines. Rubber hoses should be replaced if they are cracked or leaking. Steel lines may have to be replaced if severely corroded, leaking or damaged. Different types of end fittings are used, so be sure the end fittings on the replacement hoses or lines match the original.
PADS AND SHOES
Pads are the friction linings used with disc brake systems, while shoes hold the linings for drum systems. The friction materials used on the pads and shoes will vary depending on the application. Many front-wheel drive vehicles (FWD) use semi-metallic pads that contain chopped steel fibers. Semi-metallic pads are designed for high-heat applications. Carbon metallic and ceramic linings are also used on many FWD applications to handle high temperatures. Most rear-wheel drive vehicles and some FWD vehicles use nonasbestos organic (NAO) pads and shoes that contain little or no metallic fiber.
Brake pads and shoes are wear items and must be replaced when wear reaches a certain limit. They should always be replaced in pairs. Replacement is also necessary if the linings have been contaminated with grease, oil or brake fluid, or if the linings are unusually noisy. Replacement pads and shoes should meet OEM requirements.
Antilock brake systems (ABS) on newer vehicles reduce the risk of wheel lockup and skidding during a panic stop or when braking on a wet or slick road. ABS components include wheel speed sensors, the hydraulic modulator (which includes the ABS solenoid valves for each brake circuit, and may also be combined with the master cylinder on older, "integral" ABS systems), the ABS control module, pump, high-pressure accumulator and pump relay. Replacement is only necessary when a component has failed. ABS systems have self-diagnostic capability, and will illuminate the ABS warning light if a problem is detected. On most systems, the ABS system is disabled while the warning light is on. This shouldn't affect normal driving and braking unless the ABS pump and accumulator also provide power-assisted braking.