How do I troubleshoot my brake booster?

Master cylinders can develop a problem where the rubber cups leak past internally. They won't leak, outwardly, and depending on how bad the problem is, the master may "stop" the car OK, or not, and may slowly, or quickly leak towards the floor. Sometimes "stomping" the pedal quickly will "seal" the cup temporarily, while often "easing on" the brake will cause it to leak right down.