So, what are the symptoms of a brake master cylinder failure? The first symptom is a warning light. Modern vehicles are models of ongoing diagnostic tests. The ECU is computer in charge of monitoring the brakes. This includes the master cylinder. If the cylinder fails, the sensors will note the decrease in hydraulic pressure in one of the lines going to one of the four wheels. When that occurs, you can expect the brake warning light to fire up on the dash.
What about older models? The answer is usually not a brake warning light. Instead, we have to go old school. The key symptom of master cylinder failure is not what you see or hear, it is what you feel. A master cylinder that is going bad is going to produce less pressure to the brakes. This will manifest in the brake pedal as a "mushy" or "soft" feeling. When you press down on the pedal, you might find the pedal slowly dropping to the floor or you might feel the need to press harder to get the stopping action you want.