What Are the Symptoms of a Bad Master Cylinder?by Jin Machina
The master cylinder is the most crucial component of your car's hydraulic braking system. Without it working properly, driving your car can be dangerous for you and the people you share the road with.
Master Cylinder Functions
A master cylinder and its reservoir hold brake fluid to operate your brakes and over time wear them down from use. A piston in the cylinder operates when brakes are applied, pressurizing brake fluid through two chambers that connect to the brake lines.
Brake Fluid Symptoms
It is normal to see brake fluid fluctuate in the reservoir as the brakes are applied. If not, then perhaps your master cylinder could be having seal or vent problems. If fluid levels don't drop as your brakes wear, then perhaps the reservoir cover diaphragm is defective and is letting air into the brake fluid, causing spongy braking.
Brake Pedal Symptoms
If you're having to press your brake pedal farther down than usual, then air may be coming in through the master cylinder's chambers, causing leaks.
If your brake pedal depresses on its own, you might have a vent port blockage within the cylinder that prevents heated brake fluid from expanding and thus applying pressure to the brake lines.