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.

[cadillac_al]

Prop valve has an issue but I have no idea why. My truck just blew a line so I pinched it until I can replace it. My pedal sounds like it's working like yours. My brake light isn't on so I was hoping my prop valve is still centered/ I still have fluid flowing to the rear brakes also. So you have fluid going to the rear brakes but no actual braking?

[KY_GMC]

Yes, fluid is flowing to the rears but the cylinders are not braking. They are expanding, but no pressure.

The prop valve is centered with centering tool.

Does the e-brake cable not working have any impact?

[KY_GMC]

The other noticeable thing is that the hose from the rear brake line makes a bend/loop upwards till it reaches the axle. Can air get trapped in there while bleeding?

[geezer#99]

What’s the masters bore?
Did you check they are the same old to new?