Do you have the truck jacked up with the rear axle hanging down? If it uses a rear height-sensing proportioning valve, that will mimic a very light load, and little fluid pressure will go to the rear brakes.
If you pushed the brake pedal to the floor at any time, it is indeed possible the lip seals got damaged in the master cylinder, especially given the age of the truck. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Pushing the pedal over half way runs the lip seals over that crud and can rip them. Typically that causes a slowly-sinking pedal, and it often doesn't show up for two or three days. If only one lip seal got damaged, that would explain why the red "Brake" warning light is on. The parking brake light switch will do that too, so to double-check, unplug the wire on the combination valve under the master cylinder. If the light turns off, it will go off once the problem is solved, but even though it's spring-loaded, it can stick until you give the brake pedal a quick jab.
Do you have anti-lock brakes? If you do, and the brake fluid reservoir ran empty, there can be air in some of the chambers in the hydraulic control unit that can only be bled with a scanner to tell the computer to command the valves to open.
Monday, February 27th, 2017 AT 12:55 AM