Master cylinder
The part that turns your push on the brake pedal into fluid pressure that works the brakes at each wheel.
In plain English
Your brakes work by hydraulics, meaning a liquid called brake fluid is pushed through thin lines to squeeze the brakes at each wheel. The master cylinder is the part that does the pushing: when you press the brake pedal, pistons inside it force fluid out to the wheels. It sits under the hood on the driver's side with a small fluid tank, called the reservoir, on top. Modern cars split the brakes into two separate circuits so that if one fails, the other can still slow the car.
Why it's on your estimate
The rubber seals inside wear with age, and once they stop holding pressure the fluid slips past them instead of going to the brakes. The usual signs are a brake pedal that feels soft or slowly sinks toward the floor while you hold it, more pedal travel before the brakes bite, a brake warning light, or a fluid level that drops. Fluid can also leak from the cylinder itself.
How urgent is it?
Don't wait
The master cylinder is a safety-critical brake part, and a failing one generally gets worse rather than better. If the pedal sinks, feels soft, or goes much lower than usual, do not keep driving the car, because stopping distances grow and the brakes can fail.
Questions to ask the shop
- How did you confirm it is the master cylinder and not air in the lines or a leak somewhere else?
- Is it leaking inside or outside, and can you show me?
- Will the brake fluid be replaced and the system bled as part of the job?
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