I think my proportioning valve....

[Big Port Jimmy 6]

has gone to the bad. When I first got the crew the brake warning light stayed on, so I unplugged it. It had good brakes, so a simple fix. I swapped the rear end for an 88 up 10 bolt to get a better gear for my ls swap. It hasnt really had good brakes since I started driving it again. I had It on the rack yesterday and noticed a fluid leak from the drivers rear wheel. I pulled the drum and the wheel cylinder was leaking. So new shoes and cylinders were in order. I cannot seem to get any pressure to the rear wheels. They will not bleed. I checked the rear line at the hose and I am just getting a dribble of fluid. I checked the master cylinder with a pressure gauge and I am getting 1200 psi with the truck running.

I did not run the master cylinder dry so the pressure differential valve should not have moved and the stem on the front of the valve will not move.

Any Ideas where to get a valve for our trucks? It's an 87 2500