Rear Brakes Locking Up

[Andronikos]

I have a '86 half ton 2wd. It has the small brakes, 2" shoes in the back and 1" thick rotors in front, power assist. The rear brakes lock up with just moderate pedal pressure and the fronts don't lock up no matter how much pedal I give it. Also the rear drums make a hell of a noise as I come to a stop. Literally everything is new. MC, steel lines, rubber lines, calipers. rotors, pads, shoes, and wheel cylinders. I have bled all 4 corners and get fluid at all, so I don't think the "proportioning valve" is an issue, but that's the only thing that isn't new besides the booster. I have about 600 miles on all this stuff and the front rotors still look almost new. What the hell am I missing? This can't be normal.

Thanks in advance

 

[Edelbrock]

If the only thing that is old is the proportioning valve, why not replace it?