MAJOR COMPONENTS & FUNCTIONS
* Master cylinder — Creates hydraulic pressure when the brake pedal is depressed. Inside are two pistons that share a common shaft but push fluid through two separate hydraulic circuits. This provides fail-safe redundancy in case one of the hydraulic circuits fails. Wear in the master cylinder bore and/or worn piston seals can allow the brake pedal to slowly sink to the floor when the pedal is held under pressure. A worn master cylinder must be replaced.

* Power booster — Most vehicles have a vacuum-assisted power booster between the brake pedal and master cylinder. When the pedal is depressed, ports are opened to expose the booster diaphragm to engine vacuum. This applies pressure to the master cylinder pushrod, and reduces pedal effort. Some vehicles have “Hydro-Boost” power brakes and use hydraulic pressure from the power steering pump to assist pedal effort to the master cylinder. On some “integral” antilock brake systems, the ABS pump and high-pressure accumulator apply pressure to the master cylinder for power-assisted braking.

* Brake caliper — A hydraulic disc brake component that squeezes a pair of pads against a rotor to apply the brakes. Most calipers have one one or two pistons, but some have four or more pistons. Most calipers are cast iron (though some are aluminum). The caliper piston(s) may be steel, aluminum or molded plastic (phenolic). 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. Worn or corroded slides or bushings may prevent the caliper from sliding, resulting in uneven pad wear. “Fixed” calipers have rigid mounts and do not move sideways. If a piston in either type of caliper sticks, the caliper may not release causing the brake to drag, pull or wear unevenly. Leaky piston seals will allow brake fluid to contaminate the brake linings, and may cause the brakes to fail due to loss of fluid. Leaky calipers must be rebuilt or replaced. “Loaded” caliper assemblies include new pads and hardware, and are ready to install.

* Wheel cylinder — hydraulic drum brake component that pushes a pair of shoes outward against a drum to apply the brakes. The wheel cylinder is mounted on the brake backing plate inside the drum. Two opposing pistons move outward when the brakes are applied to push the shoes against the inside of the drum. Leaky piston seals will allow brake fluid to contaminate the brake linings, and may cause the brakes to fail due to loss of fluid.

* Disc brake pads — These are the friction linings in a disc brake. There is one pair of pads for each front wheel. The pads are squeezed against both sides of the rotor when the brakes are applied. Various types and grades of friction materials may be used for the pads, including “semi-metallic” linings (which contain chopped steel fibers and are good for high-temperature applications), “nonasbestos organic” (NAO) linings (which contain little or no steel fiber), and “ceramic” (which contain ceramic fibers and particles). Replacement pads should be the same type or better than the original. Ceramic pads are a good upgrade for NAO applications. Ceramic linings are quiet, low-dusting and typically produce less rotor wear than semi-metallic pads. Pads must be replaced when worn to minimum service specifications, or if contaminated by brake fluid or grease.

* Rotors — The cast iron disc in a disc brake system against which the pads are squeezed to apply the brakes. The rotor is mounted on the hub or axle, and most can be removed once the wheel is off (except for “captured” rotors on some import cars which are mounted on the inside of the hub and require disassembly of the hub and knuckle). The rotor’s job is to create friction when the brakes are applied, and to absorb and dissipate heat. Most front rotors are “vented” with internal fins to help cool the brakes. Rotors can wear unevenly and develop hard spots and thickness variations that cause pedal pulsations when the brakes are applied (“warped” rotors). Rotors should be resurfaced if the surface is rough, grooved or uneven, or replaced if cracked or worn to minimum specifications.

* Drum shoes — Inside a drum brake are a pair of shoes (primary and secondary) with friction linings that push outward against the drum when the brakes are applied. Most shoes have NAO linings, but some may be semi-metallic. The rear shoes generally experience less wear than the front pads, so they may not be replaced as often. Shoes should be replaced if they have been contaminated with grease or brake fluid, or when they are worn to minimum thickness.

* Drums — A cast iron shell that is part of a drum brake against which the shoes rub to apply the brakes. Like rotors, drums can wear unevenly, develop hard spots or crack. Drums should be resurfaced when shoes are replaced to restore the friction surface, or replaced if the drum is out-of-round, bellmouthed or worn to maximum diameter.

* Drum hardware — Includes return springs, holddown springs, self-adjusters and other cables, clips or springs used in the brake assembly. Springs weaken with age and from heat, and self-adjusters can become corroded and stick. Recommend new hardware when the brakes are relined.

* Brake hoses & lines — Used to route hydraulic pressure from the master cylinder to the brakes at each wheel. Rubber hoses should be replaced if cracked, bulging, damaged or leaking. A hose failure will cause brake failure. Steel lines are also used, and may become corroded with age. Different types of end fittings are used, so be sure the fittings on the replacement hoses or lines match the original.

* Proportioning valve — Reduces hydraulic pressure to the rear brakes to maintain proper brake balance. Usually located on or near the master cylinder. Some vehicles (trucks and minivans) often have a “load sensing” proportioning valve attached to the rear suspension to vary brake pressure according to vehicle load.

* Pressure Differential Valve — A safety switch that turns on the brake warning light if there is a loss of pressure or fluid in either hydraulic circuit. Usually located near the master cylinder.

* ABS components — These include wheel speed sensors, hydraulic modulators, the ABS control module (which also has Traction Control and/or Stability Control functions on some vehicles), ABS pump and high-pressure accumulator. Replacement is only needed if a component has failed. The ABS system has self-diagnostic capability and will turn on the ABS warning light if an electronic fault is detected. The ABS system is usually disabled when the warning light is on.

RELATED ITEMS
* Brake fluid — The hydraulic fluid within the brake system. There are several basic types.

* Brake service tools — Tools that may come in handy include a brake shoe adjuster, spring pliers, caliper piston tool and wheel cylinder piston clamp.

* Brake bleeding equipment — Needed to remove air from the brake lines. Professionals use power bleeders to save time.