if these rear calipers are parking brake actuated, make sure the caliper (not the cable) is adjusted out and the disc is tightened flat to the hub while bleeding/testing.
The shuttle valve will trip due to a front:rear pressure difference, such as a leak. If one end builds up pressure and the other has none (fluid movement with no resistance) the valve will trip. Once the valve trips, the pedal will get hard if the other brake circuit is good. The bad circuit will have no brake action at all. No fluid movement translates to no pedal movement.
