Diagnosing Common Brake Master Cylinder Problems

A brake master cylinder still attached under the hood of a Ford Mustang.
  • 1-20 hours
  • Intermediate
  • 0-500

Troubleshooting problems with your brake master cylinder requires some basic knowledge of car mechanics. You may never have replaced or checked this part before and if you are new to the mechanical side of motoring, you will need some good suggestions on what to listen and look for. Here are some methods you can use to diagnose the more common problems associated with this part.

1. Check the Brake Pressure

One of the major indications that your brake master cylinder is failing will be a lack of brake pressure when you depress the appropriate pedal. If you sit with your foot on the pedal and press it a few times, then keep it pressed right to the floor, it will let you know how soft it actually is. Very soft brake pedal movement is a direct result of a brake master cylinder failure. Master cylinder rebuilding is not easy and the best option is replace with a rebuilt or new one at this point.

2. Check Brake Fluid Levels

Low brake fluid levels in the master cylinder will cause braking problems and a soft feeling in the brake pedal when you depress it. If you find no signs that the fluid levels are low or leaking then you can look to the failure of the master cylinder as the main cause of this issue. Make sure the fluids are secure in the reservoirs and closed correctly and the lids are firmly in place.

3. Assess the Metal Brake Lines

brake master cylinder

The metal brake lines that go to the brake master cylinder can rot and develop a leak. Fluid can also leak through the O-rings inside the cylinder. If this is so, you will not see particularly visible signs of brake fluid escaping, but if you grab a clean rag and run it along the brake lines you will be able to detect if any wet fluid is escaping. If there is no sign, then again, it points to problems with the brake master cylinder.

4. Evaluate the Master Cylinder Grommets

There is a plastic reservoir attached to the master cylinder that holds the brake fluid. That reservoir is mounted on to the metal part of the master cylinder by rubber grommets. These will allow for some level of regular movement caused by the brake pedal and pressures of the fluids. However, they can weaken, break down, and then leak fluid. You will be able to see visible brake fluid leaking out in this case.

5. Final Conclusions

If you have thoroughly checked all the exterior components and have found no signs of anything that points away toward problems with other parts, then you can safely conclude your brake master cylinder is failing or has already failed. Putting all the above possibilities to the test will allow you to eliminate other possible causes of any symptoms which emulate the same behavior as a faulty brake master cylinder.