Do they actually release while you're driving it? My '54 2 ton did that-I'd have good pedal to begin with,and it'd get better as I went until the brakes started locking up. I'd crack a bleeder a little and all's well again,for a while.I checked the pushrod adjustment,it was fine. I backed it off a little and that made no difference. I replaced the Master cylinder and the new one did the same thing. My solution was to build a bracket and install a mid 70's GM one ton truck master cylinder on it (Same diameter cylinder,but dual reservoir.) ,with an adjustable proportioning valve to reduce the pressure to the front wheels. (I didn't bother with the residual pressure valve,but it sounds like a good move. I'll probably do that too.) My brakes are so much better now,I don't even need a brake booster.
Speed
