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?

[Dead Parrot]

e-brake not working should have no impact on the hydraulic brake system. Not saying impossible but unlikely.

If you installed new rear drums, you will need to do an initial adjust on the rear shoes. Possible they are so far from the drums that there isn't enough movement from the cylinders to get braking action. Once you get the rears close, the traditional way to do the final adjust is to go in reverse about 10mph and stomp on the brakes and let the auto-adjust do its thing. Repeat several times.

Make sure the rear shoes are installed correctly. IIRC - short shoe goes to the front.

[KY_GMC]

Thanks for the replies.

We have installed the pads correctly and adjusted them out as far as we could and still be able to get the new drums on. These new drums don't have the adjuster windows in them!

The vehicle is not driveable right now so we can't do the backup adjusting.

The master cylinder appears to be the same as what was removed. Hard to tell without disassembly.

Still wondering if there is an air pocket in the hose that goes from the rear line to the axle T-fitting. The hose actually makes a loop up from the frame line and then back down to the axle. Gonna disconnect the hose from the T-fitting and point it up and see if we bleed air out of it.

It shouldn't be this difficult to bleed brakes!!!

[geezer#99]

Are we to assume he has a factory booster and pedal assembly?

[Dead Parrot]

It shouldn't be but sometimes can be difficult. Frustration avoidance tip - make sure you keep the MC mostly full of fluid.

Even a new MC can be bad. If you eliminate everything else, try a different new MC.

The adjuster window should be in the backing plate. Not all have one. Sometimes it will have one but it isn't knocked out.

2nd the checking for flow and air from the rear hose.

[KY_GMC]

Originally Posted by geezer#99

Are we to assume he has a factory booster and pedal assembly?

Yes, as far as well can tell if is original.

When we originally started bleeding the system after replacing lines, prop valve and wheel cylinders, we had the soft pedal issue. Figured it was the master cylinder and then decided to replace it. Still getting the same soft pedal issue with the new master cylinder.

We'll try bleeding the T-fitting tomorrow once it stops raining.

[geezer#99]

I’d be inclined to suspect the prop valve.
Are the lines to it from the master the same as original?

[KY_GMC]

Yeah, we replaced the lines to the prop valve with like original. All lines came from Inline Tube.

We are on our second prop valve. Inserted the centering pin in the valve before installing on the cross member. Had to modify the pin to insure it seated all the way down. The plastic centering pin should have thread relief at the start to allow it to thread all the way down.

[geezer#99]

Maybe you shouldn’t modify the pin.

[KY_GMC]

The pin in the tool was not going all the way down into the bore. We modified the threads (not the pin) so that it would screw all the way down and prevent the piston from moving.