I am working on a 78 ford f150 4x4.

The rear drum brakes keep locking up on the truck. I have replaced everything on this truck from front to back. the prop valve was the only used part put back in the truck and I took that out of a 79 parts donor truck that I recently stopped driving. the brakes bleed fine but after 12 to 15 pumps of the pedal the brakes shoes are stuck to the drums. if I open the bleeder screw on the wheel cylinder the brakes will compress back to normal but keep sticking time after time. not sure what else I can check. any advice or help on this would be great.

 

First off, welcome to FTE.
I would go back to basics first.
Are you sure you reinstalled the rear hardware correctly.
It's easy to mess up.
Find a good pic of it, like in a shop manual.
Second, take another look at that prop valve. It could be filthy inside and hang up.
Brake problems are a pain because it's should be such a simple system.
Good luck, I'm sure more folks will chime in.

 

At first i would of said you put the auto adjusters in backwards or on the wrong side, but since you can bleed off the system and it is fine for a minute, I don't think that is it. It sounds hydraulic. So I would look at the prop valve and master cylinder. Allas I am not a brake expert, but sure someone will chime in.

 

Did you replace the brake hoses as well?
These go bad on the inside, they collapse and prevent the fluid from returning to the master cylinder.

 

The only thing that will causes this is fluid trapped in the hydraulics, not being allowed to return upstream. This can happen at the brake hose as previously mentioned, or if the master cylinder pushrod is not allowing the master to fully return. Not enough pushrod clearance can hold both chambers on the M/C or just the rear chamber, all depends on the M/C design.

The next time they lock back off the M/C retaining nuts and see if that also relieves the pressure. If it does the pushrod needs to be set. If not it's the drop down brake hose at the rear axle.

 

Are you adjusting rear brakes too tight ?Possible that secondary and primary shoes put on backwards causing a hang up problem after hitting brakes ?

 

I also would check the rear hose . Then master cyl.

 

All of the ones I've seen can be adjusted pretty easily, rather than replaced.

I'm sure there's a set of instructions that tell you proper measurements for it somewhere out there, but I've never seen 'em.

 

That was a mistype by me trying to do too many things at once. Practically all rods have a screw adjustment at the end. Some aftermarket masters are not machined as tight on the tolerances so it often with them that a readjustment needs to take place.

 

This is the adjustment procedure from the Ford shop manual.It shows how to make a gauge to set the adjustment.

 

That's awesome. If I had even a semi-stock vehicle, I'd get that manual.