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Brake Component: Brake Master Cylinder - Houston's Premier Auto Repair Facility

Services > Car Brake Service > Vehicle Brake System Parts > Master Cylinder

Function: Converts brake pedal pressure into hydraulic pressure for brake system operation.

Signs of Wear: Loss of brake pedal height, spongy or hard-to-press brake pedal, evidence of fluid leakage on or near the master cylinder.

The brake master cylinder assembly is a metal cylinder containing two pistons with seals and springs, with a plastic fluid reservoir located on the top of the metal cylinder. The brake master cylinder is usually located at the back of the engine compartment on the driver's side of the car.

When the driver pushes on the brake pedal, a rod from the pedal to the brake master cylinder forces the pistons inside the master cylinder to compress the hydraulic brake fluid. The pressurized fluid in turn forces the brakes on each wheel to apply.

Leaking seals inside the brake master cylinder can result in the brake pedal moving to the floor when the brakes are applied. A small leak normally makes the brake pedal slowly drift to the floor when sitting at traffic lights with the brakes applied. A major leak will cause the pedal to move to the floor quickly with the brakes failing to apply.

When there is a leak somewhere in the brake system, either in a hose, line, caliper, or wheel cylinder, the brake master cylinder is unable to pressurize the brake circuit. This video shows a crack in the metal line exiting the master cylinder. When a leak like this occurs, the brake pedal will often move much further than normal before the remaining functioning circuit is applied. As fluid continues to leak, the fluid level in the reservoir will start to fail, and the problem should be repaired before driving the vehicle any further.