Today I changed the brake master cylinder on my A3 because the brake pedal occasionally went to the floor. The brake fluid was black, so assumed it must have been the seals in the master cylinder. After changing, I bled the brakes in the normal way, started with the O/S/R wheel, then N/S/R, then N/S/F then O/S/F. However no matter how hard I try, I cannot get a good pedal. Am I missing something???