65 C10 brake bleeding issue

[jayoldschool]

Replaced both rear cylinders today. The master had run dry because the left rear cylinder had failed as I was driving home last fall. I topped it up before winter, and the fluid was all the way down before I got to work today. Topped it up again and started to bleed. Got a bit of fluid out of both cylinders. Wasn't making much progress until I asked my helper to push a bit harder, then got a BIG rush of dirty fluid. I could actually hear it going through the rubber line and the T fitting on the axle. Thought I was almost done... until I got no more fluid out. Reservoir is full. Brake pedal is high, and hard. Like I can stand on it with full force and there is only about 2-3" of travel. Nothing comes out when opening the bleeders out back. I've gone back and forth between both. I've left the right rear bleeder cracked to see if it will gravity bleed.

Any ideas what's going on? I haven't felt one like this before (but I've never had manual brakes). The pedal was going right to the floor before filling it up.

Theories:
- I broke something in the master
- there is a chunk of something blocking a line
- the rubber hose over the axle failed on the inside creating a blockage
- aliens

I look forward to your thoughts!