Can't bleed air out of rear brakes

I've got an odd issue with my brakes. I can't seem to get all of the air out of the rear brakes. I'm doing the 2 person pump brake method. I thought we had all the air out, so I went to the fronts and they bled like typical. Pedal went half way down, so we checked the rear again and more bubbles. Now it seems like the bubbles never end. No visible leaks. Sometimes when my wife pumps the pedal, a lot of fluid comes out, sometimes very little.

Set up is: Aluminum master cylinder (rebuilt, just installed), SSBC adjustable proportioning valve, late model discs in front, drum brake distribution block, and Ford rear discs (Explorer axle).

I've tried holding the rear calipers up and shaking to make sure they are the highest point, doesn't seem to matter.

I'm wondering if the master I got is bad- that seems to happen to a lot of people.

What do you guys think?