How a brake system works in brief
When you press the brake pedal, you are not squeezing the brakes directly. Your foot pushes on a hydraulic system, and that system pushes friction material against a part that turns with each wheel. Friction turns the car’s motion into heat, and the car slows down.
Most modern cars follow the same chain: pedal, booster, master cylinder, brake lines filled with fluid, and then either disc or drum brakes at each wheel. An anti-lock braking system (ABS) sits on top of this to stop wheels from locking up in a hard stop. Layouts vary by vehicle, so treat this article as a general guide and check your owner’s manual for specifics.
The parts you control and the hydraulic side
- Brake pedal. The lever your foot pushes. It moves a rod that acts on the brake booster and master cylinder.
- Brake booster. Multiplies the force from your foot so stopping takes modest effort. Many cars use engine vacuum for this, while others use an electric pump or hydraulic assistance.
- Master cylinder. Turns the pedal’s mechanical push into hydraulic pressure. It usually has a fluid reservoir mounted on top, and it typically feeds two separate circuits so that a leak in one does not remove all braking.
- Brake lines and hoses. Steel tubes carry fluid along the car, and flexible rubber hoses connect to the wheels so they can move with the suspension.
- Brake fluid. The liquid that carries pressure. It does not compress easily, so pressure from the pedal reaches the wheels. It also absorbs moisture over time, which is why it needs replacing periodically. See how often to change brake fluid.
Disc brake parts
Disc brakes are used on the front wheels of nearly all modern cars, and on all four wheels of many.
- Brake rotor (disc). A metal disc that spins with the wheel. This is the surface the pads squeeze.
- Brake pads. Blocks of friction material on steel backing plates. They sit on either side of the rotor. Pads are typically made from ceramic, semi-metallic or organic compounds, each with different noise, dust and wear traits.
- Brake caliper. The clamp that straddles the rotor. It contains one or more pistons that are pushed out by hydraulic pressure and press the pads onto the rotor.
- Caliper bracket. The piece that holds the brake pads in position is generally the caliper bracket (also called the caliper carrier or anchor bracket), which is bolted to the steering knuckle or axle. The pads slide in the bracket’s abutments, often with metal clips between them.
- Hardware. Guide pins, rubber boots, anti-rattle clips and shims. They look minor, but they let the caliper slide freely and help keep noise down. Worn or dry hardware is a common reason pads wear unevenly or squeak. See why brakes squeak.
Drum brake parts
Some vehicles use drum brakes at the rear wheels, particularly smaller or older designs and many trucks.
- Brake drum. A hollow metal drum that spins with the wheel.
- Brake shoes. Curved friction pieces inside the drum. They push outward against the drum’s inner surface.
- Wheel cylinder. A small hydraulic cylinder that pushes the shoes outward when you brake.
- Springs and adjusters. Return the shoes after braking and keep them close to the drum as the lining wears.
Parking brake
The parking brake (also called the emergency brake or handbrake) works independently of the hydraulic pedal circuit and is mainly meant to hold the car still when parked. On many cars it is a cable that applies the rear brakes. Many newer cars use an electronic parking brake, in which a small motor on the rear caliper or a related actuator does this job. Electronic systems may need a scan tool or service mode before the rear pads are changed, so this is worth mentioning to whoever does the work.
ABS and electronic parts
- Wheel speed sensors. Monitor how fast each wheel is turning. For other sensors on the car, see types of car sensors and what each one does.
- ABS control module (and hydraulic unit). Compares wheel speeds and, if one wheel is about to lock, rapidly adjusts pressure to that wheel.
- Warning lights. The brake and ABS lights on the dashboard tell you when the system has detected a problem. Their meaning depends on the vehicle, so check your owner’s manual and have the system inspected if one stays on. Our dashboard warning lights guide covers the common symbols.
Which brake parts wear out and need replacing
Not everything in the system wears at the same pace.
- Wear items: brake pads and shoes are designed to wear and are replaced most often. Rotors and drums wear more slowly and are often replaced, or resurfaced where allowed, when pads are changed.
- Periodic service: brake fluid, and caliper hardware that gets cleaned, lubricated or replaced during brake work.
- Replaced if they fail or deteriorate: calipers, wheel cylinders, hoses, lines, the master cylinder, sensors and the booster.
For typical mileage and cost, see how long brakes last.
Common brake problems and the parts behind them
There is no single official list of problems, but most complaints trace back to a few causes:
- Squealing or squeaking: worn pads, dry hardware, glazing, or surface rust on rotors.
- Grinding: pads worn down to the backing plate, or debris caught in the brakes. Get this checked promptly.
- Soft or spongy pedal: air in the lines, old fluid, a leak, or a failing master cylinder.
- Pulsing pedal or shaking steering wheel: often uneven rotor surfaces.
- Pulling to one side: a sticking caliper, uneven pad wear or a collapsed hose.
- Brake or ABS warning light: low fluid, a sensor problem, or a fault in the system.
- Burning smell or heat from one wheel: possibly a caliper that is not releasing.
Any problem involving pedal feel, leaks or reduced stopping power is a safety issue. Have a qualified technician inspect it rather than driving on it.
FAQ
What are the six basic parts of a brake system?
Sources count differently, but a common simplified list is the pedal, booster, master cylinder, brake lines with fluid, calipers with pads and rotors (or wheel cylinders with shoes and drums), and the parking brake.
What is the piece that holds the brake pads called?
Usually the caliper bracket, with the caliper itself clamping the pads against the rotor.
What parts of brakes need to be replaced most often?
Pads (or shoes) first, then rotors or drums. Fluid is replaced on a schedule, and other parts only if they wear or fail.