76 C10 Brake Bleeding Issues

[KY_GMC]

Helping a young man bleed the brakes on his 76 C10.

He has replaced the following:
Master Cylinder - bench bled
Proportioning Valve - w/ centering tool inserted
Front Calipers
Rear Cylinders and pads
brakes lines to rear, new T-Hose, and rear axle lines
Brakes lines to Prop valve

When trying to bleed the brakes, the pedal is soft for the first half of the stroke and then gets firm. Opening the rear bleeders does not allow the pedal to go to the floor. Opening the front bleeders does. We are getting good flow through all bleeders. The rear brakes are not building pressure no matter what and the pedal never gets hard once you pump it a few times. The rear wheels are not braking but the fronts are.

One thing to note is that the E-brake is not working. Does this have any affect on bleeding?

Any recommendations on how to bleed or what might be causing the soft pedal would be appreciated.