I bought an adjustable proportioning valve off ebay for $25 as I recall. Looks identical except chrome and no MP & Wilwood logos.
I disagree w/ fklskv. Smaller cylinders in the rear would give less braking in the rear.
I also agree that sticky shoes is the most likely culprit, since it didn't lock up previously. Have you checked them? My fairly new 82 Chevy S-10 got grabby in the rear and I found a cylinder leaking on the brake shoes. It was also rusty inside. No telling what GM installed for brake fluid. Imagined an open drum where workers dumped their drinks. I would also bleed all calipers and cylinders. I once couldn't bleed a caliper on a Dodge Aries, even with the bleeder removed. Finally forced rusty gunk thru, then they worked. Nothing but silicone fluid for me since (some FABO's will scream).
