rear brakes won't bleed

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?