rear pressure low in system HELP

What you describe "in the master" cannot happen unless the front piston is already bottomed.

Is there ANY possibility that instead of a pressure problem, it's a rear brake problem? They drum? disk? shoe adjustment?
EDIT Missed the fact that you have rear disks EDIT

Might be time to "rig" a pressure gauge so you can be sure it's actually a pressure problem

Otherwise, I'd be "bench bleeding" the master, and if that does not work, tearing the master apart to see what.................

You might also get someone, or a mirror, so you can see fluid action in the master. You should be able to apply the brakes, then see a EDIT small amount of returning fluid from both reservoir ports

Maybe the front piston is missing the return spring, or has a defect, sticking in the bore???