Since I replaced my rotors and brake pads before last ZuZoo I have been a little worried about my brakes. They seemed to work just fine on the street, but in Moab I noticed that my pedal would hit the floor after standing on it for a while to creep down some obsticles.

I've bled and re-bled the brakes and there doesn't seem to be any air left in the system so I think I'm good there. The pedal is firm and I can panic stop with the best of them, but if I'm sitting at a stop light and I really press down on the pedal it slowly drops all the way to the floor. Since it stops fine under normal conditions, and only drops the pedal when I stand on it for a few seconds, I'm wondering if my master cylinder is dying. The fact that the pedal drops tells me that maybe the seals might be leaking and fluid is pushing past them back into the reservoir under constant pedal pressure.

Opinions?