I have a 69 roadrunner with front disc rear drum using the 73-76 a body setup. I was driving a week ago and the pedal started to fall when I hit the brakes, and the brake light would turn on. I drove it home, and checked for brake fluid leaks. There were no external leaks I could see. The symptoms appeared to be a bad master so I ordered a new one, and put it in. After bleeding the master the pedal is firm with the car stopped, but as soon as you drive one or to stops in the pedal will fall and the light comes on. It comes and goes but its hard to stop the car when its doing it. Anyone have this happen before?