Can't bleed air out of rear brakes

I'm having the same problem--replaced the power brake booster (vacuum leak). Bought the assembled with new master cylinder since, why not...

Bench bled the MC with scrapyard brake lines fed back into the reservoir.

2-person bled the system in proper order RR, LR, RF, LF and the pedal felt fine until I turned the car on...pedal to the floor.

Gravity bled the system with same results.

Jacked the car up while wife held the brakes. Fronts are locked and rears are free.

I haven't swapped to the old MC (which worked fine, btw) but that's my next step.

Thoughts?

1973 Swinger /6-225 front disc/rear drum