Hi, Im a mechanic, and it sounds like you might have a contaminated master cylinder. If the brake fluid is really dark like black coffee,, and the rubber on the cap is all warped you have a hydraulic failure in the master cylinder. Some seals are starting to go bad and they move. this might exlpain your pedal getting better again. Nothing ever fixes itself. It just starts working again, but they will fail again if you dont look into it. Be safe and have a licensed mechanic look at them. ASE mechanics are the best.
Donald
July 2005