Thanks so much for the quick responses. I guess I should have been a little more specific with my question. I filled the ABS unit with DOT 4 fluid, slowly pumped the rear brake lever, but no fluid appears to be getting to the rear master cylinder, since no fluid comes out of the caliper bleed valve when I crack it open. The rear brake will not even begin to "pump up". Could this be an air lock problem brought on by 2 1/2 years of setting?