I installed some new brake hoses on the truck and decided to completely replace the brake fluid. When bleeding the rear brakes (both sides), the pedal would barely move. It was almost like the bleeder valves weren't open. However, they were open and a little brake fluid was coming through.
Moved to the front and both sides bleed just fine, with the pedal going all the way down to the mechanical stop.
Before I started the job, I replaced the switch with one of these to make sure the pressure differential valve didn't move.
Then when I had the problem, I removed it and re-installed the switch. The switch is closed causing the dash warning light to turn on. Stepping on the brake pedal doesn't help. So, it seems like the valve is stuck in the rear position. But the light had never come on solid before. It had flickered somewhat, but replacing a worn out switch cured that.
***See post #6 below to see what I think was causing the switch to close.
Have to say that for some time now I've felt the truck's front brakes were doing most of the work (more than normal). And when I pulled the rear drums, there was only a small amount of brake dust and the old shoes looked almost new.
The prop valve is from a 71-72 truck and may be the original. Could that be my problem, or might it be the master cylinder?
I have a brand new aftermarket prop valve, but have been reluctant to use it, given all I've heard about leaky seals.