Durango Brakes Won't Bleed

Bought an 01 Durango RT a while back. It's rough, but it was cheap. We had to replace the front calipers because one locked up, basically had simple project turn into a super long, super painful project, but it's mostly unrelated. After we did the front brakes, the pedal was never really back, it worked but had way to much travel. Finally got around to fixing the pedal, replaced the back brake cylinders and all the hardware back there. Bled and bled and bled. No pedal. Replaced the master cylinder. Bench bled, and then bled and bled and bled. No pedal. The car has rear abs only, not all wheel abs as far as we can tell. We've put something like a gallon of fluid through this and it just wont bleed. Any ideas?

I know it sounds weird but.
Have you tried bleeding the brakes with the engine running?