For some reason my rear brake keeps losing pressure at what seem to be random times. I rebuilt the master cylinder last spring and have bled them several times. What is throwing me is that they will hold pressure for hours/days and then suddenly be gone (of course just about the time I really need them). Here is what I have done so far: rebuilt the master cylinder with aftermarket parts. Bled them several times. Rebuilt the master cylinder again, this time with Yamaha parts, bled them. Replaced the rotor (thought may it was warping when it got hot). At this point I am thinking that maybe the fluid is boiling. I used fluid (at least on the last rebuild) that I bought off the shelf at the parts store (not a name brand). I don't see any leaks, but I did notice that the top of the master cylinder where the cap bolts on was a little damp.... Any ideas...?