Proportioning Valve Problems

B4 I went after a porportioning valve I would check a few other things.as a rule porportioning valves distribute fluid/pressure thru fixed orifices and very very rarely give any probs like this. More likely it is something to do with the master cylinder not releasing the rears or the rubber hose to the rear internally balloning and not letting the fluid return into the system. I have seen more hoses keep pressure on a brake than stuck calipers, porportioning valves,pinched lines, (have you jacked the car up and maybe pinched a line,)

way to test is go drive car and start cracking lines all the way back to the master when yo quit finding pressure go back to the last place you did and you found the problem