I'd try it this way. Take the rear brake line off the hose to the rear axle and have someone push the pedal slowly. Watch to see how fluid is coming out. Nice steady stream? Got problems somewhere between the hose and the wheel cylinders. Trickles? Then the problem is from there forward. Unhook the line from the block, have someone push the pedal slowly again. Steady steam? Then the problem is in the hard line going back. Trickle, then the problem is obstruction in the block or in the line between it and the master, or the master itself. Keep narrowing things down like this. You'll need some more fluid, but it can help narrow things down, and you're trying to bleed the system anyway.