[Rust_never_sleeps]

Rears fought me the whole way. Some things that do help:
Good condition rear hose and wheel cylinders
Properly adjusted rears (if they have too much travel, they do something mysterious and block the flow)
Properly centered and pinned shuttle valve(diff pressure valve) in the combination valve. Verify this visually. Re-center it by cracking a front bleeder or pulling it back with a pick, not by slamming on the brakes.

I got no love using vacuum bleeders, but had good luck with the Motive tool. IDHAFC why one worked and not the other