I have a 1991 F150 4x4. Until yesterday my rear brakes worked great. All of a sudden the ABS light came on. I took it in to the mechanic. They believed it to be the switch, and replaced it, but the light remained on. They think now it is the switch within the master cylinder. Other than that they did nothing to the truck. Theoretically even if the ABS isn't working the hydraulics should. When I took off this evening for a horse show with trailer in tow. The brakes worked for about 30 miles, but I smelled hot brake odor off and on. ( I don't use my brakes much when towing, and tend to let the engine slow me down.)All of a sudden, the brake pedal goes to the floor, and I have to pump them for or five times to make them firm. Thank goodness for the trailer brakes. I stopped, got someone else to come and get the horses and drove home. We pulled the fuse for the ABS thinking it might be the "dump valve". Again the brakes worked fine for the first 30 miles, then all of a sudden pedal to the floor, and when I got home the rear wheels smelled like hot brake, and were extremely hot to touch. The fronts are fine. There doesn't appear to be any loss of brake fluid. Any suggestions? Is the master cylinder going. There is plenty of brake pad left.

Morgan