I had a similar problem on my 46 coupe. In my case it turned out to be air trapped in the rear line. I could not get it out by the traditional pump and bleed method. I finally built a pressure bleeder and that was able to move enough fluid in a steady flow and get all the air out. This might not be your problem, though. Just a thought. Like you, this was the first car I couldn't get to bleed in the normal way

I would have plumbed the residual valves closer to the MC and used only 1 for the front. Also, I have a prop valve in my rear brakes and it is not needed. It is set to max pressure. I have Ford Explorer disc brakes on the rear. Between the smaller disc size than the front, smaller caliper bore and difference in tire size, the bias is built in. As has been said, you may have built in brake bias that simply won't allow the rears to lock. In my case, I know the rears are working because the rotors show wear and the pads appear to be wearing.