No fluid to rear drums: WTF?!?!

[BigTopGT]

I recently picked up a decent 71 C10 (LWB, 350/350 auto) with a soft brake pedal.

I got her home, replaced the booster, the master, and the Proportioning Valve, then tried to bleed. (the master ws bench bled)

After several days of messing with it, I just CAN NOT get fluid to the back brakes. I even went so far as to take it to a local Meineke and had them try to use their 3 bar power bleeder, but THEY just called and told me they can't get fluid to the rear, either.

I just don't get it.

I mean, it's not a complicated system, and I replaced everything except for front calipers and rear wheel cylinders, so what the eff can this be?

One thing of note: if you crack the rear line at the bac of the Proportioning Valve, yoh don't get any fluid, but I'd you crack it at the master, it leaks.

Could I simply have a brand new, but still defective, proportioning valve?

I'm at the end of my rope here, guys... Someone is coming to pick up my 67 today and I need a truck to drive. :/