When I changed my rear pads on a 92 CR500 last weekend I bled the rear brakes. The fluid came out black, like used motor oil. The fluid in the reservoir was clear, so I'm guessing that the black came from the inside of the caliper cylinder. There are no leaks in the system. The fluid was in there about 1.5 years, and I ride at a motocross track pretty regularly. Is my rear caliper worn out? Any suggestions?