Gotta bleeder valve question for you. If the valve goes bad, will it not let the break fluid through when bleeding? I checked my lines that I replaced and they're not leaking, but the line could also not be fitted to the housing.
Only thing I can think of on bleeder valves is the ball gits stuck. Also, double check that yer LSPV (load sensing proportional valve) is working. I would just disconnect the rod that attaches to the rear axle housing and push it towards the top of the bed. That should allow you to git a full flow to bleed yer rear brakes. You can tie it off there if you don't have a helper Don't fergit to bleed the LSPV out as well...and don't fergit to hook the LSPV rod back up when done with the back brakes!