Do prop valves go bad?

[TugOwar]

New master cylinder installed today, replaced the leaking front brake hose, bled all lines. Decided to just pull all the drums and have a look. Actually looks pretty good, shoes about 1/2 worn, drums smooth and don't have a big lip on the edge. No leaking wheel cylinders, actually looks like things were gone through and freshened up not long ago.

Put everything back, adjusted all the brakes, bled everything again, took her for test drive, Still have the same BS!
Better pedal, quicker response, but still feels like I'm compressing air somewhere. I can hit the brake and feel the brakes start to grab but the pedal slowly goes down, hit the pedal again and everything tightens up, good pedal, then if I still have enough speed the rear brakes suddenly lock up and slide.

I can take the top off the MC and remove the bleeder from each front wheel and I have a good, fast flow of fluid.
I can do the same to the back and I get a flow of clean fluid but it's quite a bit slower flow compared to the front.

Based on this here is my theory: The rubber brake hose at the rear end is breaking up on the inside, like front ones do sometimes. I can imagine the inside of the hose folding up against pressure, then suddenly giving way and letting the fluid rush to the rear wheels. Maybe it could even hold air behind it because of restricted flow?

I don't know, I'm open for suggestions here guys. I don't expect perfect brakes from a 30+ y/o truck but I do expect a good, tight pedal and all four wheels to stop at the same rate. These big a** drums and wide shoes and power booster have the potential to provide great stopping power.