rear brakes won't bleed

Just Send It said
Did you bench bleed the master cylinder prior to installing? If you're bleeding by the pump-and-hold method, there could be a lot of air trapped in those new lines. It could take a while.

If the MC has been primed and you start bleeding the wheel furthest from the MC and systematically work to the front wheels, the air will have to push out eventually. Or you're sucking air in from somewhere.

Is your distribution block made for front/rear disk brakes or front disk/rear drum?

The MC was bench bled prior to installing and I am doing it the old fashioned way of pumping and opening the bleed valve starting at the pass side rear. The db was listed for disc/drum brakes, I did have a couple of the fittings that were leaking at the db and the t block but tightened them up to cure that. On my initial bleed attempt I didn't get as much air as I thought I should have, I have tried bleeding them about a half a dozen times with the same results.