Originally Posted by ol' grouch

I'm trying to picture in my head what's happening here. I don't think I've ever seen the button pop up on a proportioning valve. I know it will as I've tuned brakes and had to pop it up by hand (well, pliers) but I've never seen one do it by itself. Are the right rear brakes working okay? Has this truck been hauled on a trailer or pulled something out of a ditch with a chain around the axle tube? I ask just to make sure there's no damage to the steel line going to the right brake. Personally, if I change or rebuild one wheel cylinder, I do the other one also just to keep everything balanced. The brake shoes are clean aren't they? If the wheel cylinder leaked, brake fluid can cause it to grab. How is the brake cable for the parking brakes? Working normally? It may be the prop valve, although that's rare. Otherwise, I'm thinking there's something simple causing your issues.