Question re: poor line pressure at rear brakes

May I please ask you guys about the rear brakes on my '77 B200 mid van, which are basically very similar to, or the same as, the brakes used on all Chrysler products. The rear brakes were working poorly so the fronts were doing almost all of the work, and the stopping was poor.

I disconnected all the rear lines, blew them out with air and replaced the rear central hose. When I pump the pedal, brake fluid flows through the lines like it should. The old hose didn't seem to be clogged (felt about the same as the new one when I blew through it).

This seems to leave the proportioning valve or the master cylinder as the culprit.

The master cylinder is an A-1 Cardone rebuild, which as we all know can sometimes be defective, while the proportioning valve is the original, 42 years old. I can probably get a free warranty replacement master cylinder from RustyRatRod's employer, if necessary. But before trying that, I need to figure out if old proportioning valves ever go bad? How do you test one?

Thanks a lot, guys.