Here's the deal : my brake pedal sinks slowly when applied. Now, I know that most of the time this is caused by an internal leak in the master cylinder. I have no fluid leak anywhere else and my fluid level is fine. The only problem here is this situation doesn't happen all of the time. Some times the brakes work fine (sometimes even for weeks at a time) and all the sudden this happens. I can still stop when this happens, it just takes a little extra pressure. It has no rhyme or reason to it happening, it just does it. Could anyone tell me what I could do to check (without dis-assembling the master cylinder). Any help would be greatly appreciated.