Brake System

Article describes how an automotive brake system works.

Brake systems are designed to slow the vehicles wheel movement through friction. There are primarily two brake types of systems, ABS and non ABS (anti-lock brake system). Both systems work on basic hydraulics and utilize a brake master cylinder (connected to the brake pedal) that supplies brake fluid pressure to the front brake calipers and rear wheel cylinder or brake calipers if so equipped.  The brake system requires hydraulic force that is many times greater than the force applied by the foot. This added hydraulic force is achieved by leverage multiplication.

Power brake systems utilize either vacuum (vacuum assist) or power steering (hydro-boost) to multiply the force without added pedal effort. When the brake pedal is forced down brake fluid from the master cylinder is transferred to the brake caliper piston, pressurized fluid is transmitted to the brake calipers or wheel cylinders.

Brake System
Typical Non ABS Brake System

A major problem to a brake system is a brake fluid leakage, if there is a slow leak in the system it will affect the brake master cylinder by allowing air into the system creating a low pedal and eventual brake operation failure. If a large brake fluid leak is present, fluid escapes, leading to a major brake failure on one half of the system (front or rear.) A master cylinder has a built in safety device, two separate hydraulic systems a primary and secondary, with separate reservoirs separating front and rear brake operation. In case of front or rear brake operation failure, one system will still be present.

Common Problems and Fixes

Today's brake systems are reliable and easy to maintain, most systems are designed for easy service and brake pad replacement. If the brake pedal becomes hard to push, there can be a possibility of a broken vacuum supply line or a failed power brake booster. It is also very important to use the correct type of brake fluid; otherwise the brake system can malfunction. This can lead to problems such as spongy feeling pedal and the brake pedal travel becoming excessive. Only the highest quality brake fluid must be used in a brake system and must never be reused and new fluid must always be closed immediately after use to avoid moisture contamination. When adding brake fluid to the master cylinder, care must be taken to not spilled fluid onto the paint of the car, brake fluid is very corrosive and will destroy the surface.

Squealing brakes is an indication of a problem with the brake system. A squealing noise can be a warning that the brake pads are worn and need replacement. Other causes can be overheated brake pads or rotors. Sometimes rust can get impregnated on the brake lining material and cause grumbling or squeaking noises. When cleaning brake components prior to service precautions must be taken. Most brake systems contain microscopic fibers that are hazardous to a person's health. Cleaning should be done in a well ventilated area, use an air filter respirator if possible for best results.

When the ABS system detects a problem, a fault code is stored in the ABS system control unit. The ABS warning light will begin to flash or stays on steadily. The ABS computer has stored a diagnostic code to help the system to be repaired. To retrieve these ABS trouble codes on older vehicles can be tricky because of the variation of retrieval methods; in this case a car repair manual is needed. On newer vehicles a diagnostic information connector is located under the dash on the driver's side; a trouble code scanner can be attached to the ALDL connector to retrieve ABS codes, similar to engine trouble codes.

After the problem has been repaired, codes should erase automatically with no procedure from the scan tool in most cases (real-time system.) When the vehicle has been moved about 6 feet and the brake pedal has been pressed for five seconds the codes should clear. When repairs have been completed the ABS, MIL (malfunction indicator lamp) should not be illuminated, if the ABS light remains "ON" there is still a problem, re-scan the ABS computer for further repairs.

The brake proportioning or combination valve is located in line with the front and rear brakes. This valve is used to monitor the brake system pressure from front to rear and warn you if a fluid pressure drop is detected. When a system failure is detected a brake warning light is illuminated on the instrument cluster.


Written by
Co-Founder and CEO of
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2015-01-06)