Usually a faulty brake master cylinder will produce a few symptoms that alert the driver that service may be required; abnormal brake pedal behaviour, contaminated brake fluid and the usual check engine light. The most common symptom to first notice is abnormal brake pedal behaviour. As the master cylinder is the component that generates all of the pressure for the braking system, if it develops any sort of problems sealing or distributing pressure, this may be felt in the pedal. Over time, with constant use, the seals inside the cylinder can wear out and form internal leaks. A bad brake master cylinder may result in a pedal that feels mushy, spongy or that slowly sinks to the floor when depressed. If you experience any of these symptoms, make sure to have the issue inspected by a mechanic immediately.