You said you replaced it already, so that leaves just two possibilities. One is something is holding the brake pedal down a little. That is almost always the brake light switch. The second is the brake fluid is contaminated with a petroleum product. The clue there is the new master cylinder would have solved the problem for a day or two but it would have occurred again. The only fix for contaminated fluid is to remove every part that has rubber in it that contacts the brake fluid and to flush and dry the steel lines, then replace the parts and put in fresh, new fluid. If any rubber part is not replaced, the contamination will leach out of it and recontaminate all the other new parts. That includes calipers, wheel cylinders, rubber flex hoses, master cylinder, combination valve, and height-sensing proportioning valve. A '79 might not have the last one.
Thursday, July 3rd, 2014 AT 11:01 PM