Rear Brakes Lock Up WAY too soon!

[69GMCLonghorn]

I have a 1-Ton with 4 wheel power drum brakes. In the 6 years I've had it, I haven't been able to solve the problem of why the back end locks up even under light or moderate braking. Here's what's been done so far:

The brake shoes have been changed about 3 times
Drums turned once (still perfect)
Wheel cylinders replaced @ all 4 corners (no leaks)
Master cylinder replaced
Axle seals replaced (I thought maybe a small amt of gear oil might be doing it)

One guy said it felt like the back end was braking like the front end should, as if the lines were swapped. I've looked at it and you couldn't do this without totally rebending the lines. It has a proportioning valve, but there's no adjustment on it.

What I've done that has helped is I installed an inline pressure valve (good old Summit Racing to the rescue!). This has helped, but now the brakes feel funky. If I really hit 'em, there's a point where the pedal just stops and won't go any farther (about 1/2 way to the floor). Could the brake booster be causing any of these problems?

I'm completely out of ideas. Anybody with more brake knowledge than me, please chime in. I'm open for suggestions.