Quote Originally Posted by SJAndrew View Post
The easiest way to diagnose this is to stand on your brake pedal. If the pressure builds, peaks, but then slowly goes away and the pedal goes to the floor, you have a bad (leaking) master cylinder (or a hydraulic leak somewhere in your system).
This is exactly what it's like. I can 'pump' the brakes to get it to stop about a third of the way down, but once I'm stopped, it slowly sinks to the floor, and I doubt it's a leak, since fluid levels have remained high, and I've never noticed any leakage in my garage.