Brake Master Cylinder Troubleshooting

Reader Question: My mechanic said that my master cylinder needs to be replaced soon. How can he tell? I brought my car in for other repairs, and I don't know if he is just trying to sell an add-on job.

Dear concerned car owner,

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So you are starting to feel uneasy about your mechanic's recommendation? How do you know if he is doing his job correctly and looking after your best interests and your safety? When a brake master cylinder begins to go bad or fails, you will notice a soft or squishy feeling when you press down on the brake pedal. When constant pressure is maintained on the brake pedal (like when you keep your foot on it at a stop light), the brake pedal will begin to sink to the floor as the brake fluid leaks internally in the master cylinder. This is called "extended travel" when the brake pedal goes farther down than normal or than it was intended to. When this occurs you will have to "pump" the brake pedal to regain normal pressure and to keep the car from moving forward because the brakes are slowly releasing. The red brake warning dash light should come on to indicate low brake fluid, or excessive movement "travel" in the brake pedal.

You will not normally see the brake fluid leaking out externally of the brake master cylinder or from the wheel areas when this pedal softness occurs, so fluid level alone isn't an indication of a good or bad brake master cylinder. The fluid will usually not be low or in need of topping off…remember the leak is internal and the brake fluid is leaking past internal O-rings, so an obvious external sign that the master cylinder is "bad" is usually not existent.

Ok, that is great Austin, but that is not happening in my case. What else could have tipped off this mechanic to a possible faulty brake master cylinder? An external brake fluid leak could be possible, and would be visible to the mechanic as he was performing his usual under hood inspection. The master cylinder usually has a plastic reservoir that holds the brake fluid, Click for example, and this reservoir is mounted to the metal part of the master cylinder by rubber grommets. The grommets allow for some movement caused by brake pedal and fluid pressures. These grommets can leak fluid, and a visible brake fluid leak can be seen with the naked eye (brake fluid is a clear liquid).

The metal brake lines that are attached to the master cylinder can leak fluid at the threaded connections, Click for example. Brake fluid can also leak from the back of the master cylinder due to these internal O-rings we talked about earlier. When this happens, the fluid will drip down the power brake booster (the booster is what the master cylinder is bolted to), Click for example. Brake fluid will dissolve paint rather quickly, so this kind of leak will usually leave a tell-tale sign of blistering, bubbled paint under the brake master cylinder.

So if you experience any of the symptoms explained above, give your mechanic a "brake." He was doing his job thoroughly and was trying to help. In any case, check the brake fluid level first and determine if fluid is needed. Check the cap and make sure it is fitted snuggly on the cylinder and is not leaking or showing any obvious signs of past leakage. Visually inspect the cylinder externally for fluid leaks, and wipe any accumulated dirt and debris from the area and the cylinder for ease of future inspections. The cylinder should only need a very small amount of brake fluid as part of regular maintenance, so if more than an ounce or two is needed to top off the reservoir, you should have your brake system inspected by your mechanic. Do not put off any needed repairs to your braking system.

I have put together some easy-to-follow maintenance schedules with more recommendations and explanations that are free to view and print out from our  website.  Maintenance Schedules.

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Sincerely,

Austin C Davis
The Honest Mechanic