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?

[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.

[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.

[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.

[straight6chevyguy]

Like Dead Parrot said, the adjuster window is in the backing plate not in the the drum on these.

What bleeding method are you using?

[KY_GMC]

Doing the two person bleed: press pedal and open bleeder fitting. Start in rear and then do the fronts.

While doing this, we noticed that the for the front, the pedal goes to the floor while bleeding, but the rears do not. And the pedal never gets hard at the top. Stays soft until about half way down.

[cadillac_al]

Well, the prop valve still has an issue but I'm not sure what to do with it. I have never had a centering tool or used a centering tool. When I have a prop valve blocking pressure to the rear I first take it for a short easy slow ride and stomp on the brake pedal mercilessly. I do it in drive and reverse. After a while it will usually come out of it. One time it wouldn't come out of it and I used a trick I learned on the net. I just opened a bleeder on the front brakes and give it a few pumps and the prop piston will move the other way. You have to be kinda careful not to go all the way to the floor because you can damage an internal seal in the master cylinder.

If I wouldn't have blown a brake line in my GF's Jeep I would be fixing my truck rear brake line this weekend and have a better feel for what you are going through. The very squeaky wheel gets the grease this weekend.

My truck pedal has no resistance for the first half of travel then it is very firm. Once I fix the brake line or maybe two on that Jeep I won't be in the mood to fix my truck this weekend. That's the only food for thought that I can add at this time.

[KY_GMC]

So, we finally got the brakes bled with a hard pedal.

The new hose assembly from the rear line to the axle lines was slipping on the crimped fitting at the tee. We could turn the hose, even though the end fitting was tight on the tee fitting. We replaced the hose and made sure that the air would purge out of it by holding level to allow all the air to pass through and then bled the system.

Thanks for all the suggestions but it turned out a faulty replacement part was the culprit.