The master cylinder stores your vehicle's brake fluid, which is used to hydraulically operate the brake system by forcing the fluid to the brakes. If the master cylinder is defective or leaking, it will cause a loss of fluid and send air into the system that makes the brakes feel "spongy" when applied. If your brakes are malfunctioning, there are steps you can take to see if the problem is with the master cylinder or elsewhere.
Testing the Brakes
The simplest first test is with the brake pedal. With the engine off, press down on the brake pedal for 10 to 15 seconds and see if it drops further down during this time span. After releasing the pedal, test it again in the same way with the engine running. If the pedal drops at any time, there is likely a leak in the system, but it may not necessarily be at the master cylinder. Be sure to check all the brake lines and the brake assemblies at the wheels for leaks as well.
Bleeding the System
Bleeding the brake system can also correct problems, which can signal that the master cylinder was not the issue. You need to raise the vehicle and remove the wheels to bleed the system; there is a bleeder valve and screw on each caliper or wheel cylinder. Attach a clear tube to the bleeder valve and open it with the screw while another person presses down on the pedal. Once fluid comes out in a clear stream with no bubbles, close the valve and move to another one. Once the system is bled at all four valves, refill the master cylinder with as much brake fluid as needed.
Inspecting the Cylinder
To inspect the master cylinder for leaks, you will need a flashlight to get a clear view around its fittings. The grommets connecting the cylinder to its reservoir are a possible spot for leaks, especially since the grommets are not excessively tight to allow some movement. The fittings connected to the brake lines can also leak, but the primary location of leaking is at the rear of the cylinder where it meets the brake booster. This is a sign that the master cylinder is defective and needs replacing. Keep in mind that brake fluid dissolves paint, so any sign of paint blisters or bubbles near the cylinder is a good sign of leaking. Once you patch up any leaks anywhere, especially at the cylinder, you will need to bleed the system as described above. Once the system is bled, try the same brake pedal test as before. If you still have a problem with the brake pedal dropping, the master cylinder is probably defective.
