Stuck on stuck brakes

[Reddeye]

I have seen several threads related to my issue but nothing seems to pinpoint my specific problem. '72 C20 Cheyenne with front discs and rear drums, and front brakes starting dragging when it got hot. Figured it was old brake hoses, so I replaced with stainless hoses and eventually replaced pads, calipers, and lines up to proportioning valve, since they were quite old. Fronts are still dragging. What's the likely culprit, proportioning valve, master cylinder, booster?

[pjmoreland]

I had a case of dragging brakes once, and it was because I didn't adjust the rod properly that pushes on the back of the master cylinder. As a result, the master cylinder piston wasn't able to retract 100% when I wasn't pressing on the brake pedal. I believe this traps the fluid pressure in the lines, preventing the brakes from releasing completely.

[Sheepdip]

Have you messed with the master cylinder or booster? The master has to be adjusted pretty close to the booster output shaft with a small clearance, if that's not right you will get the dragging effect you are describing. Could also be the master is not returning all the way on it's own because of age.

[Reddeye]

I haven't touched anything further up the chain from when I connected the new lines to the prop valve. I'll try playing with the master cylinder rod. At this point, and based on the age (to the previous owner, "just barely replaced" = 20 years) I'll probably end up replacing it all.

[Asshat]

You sure it’s the front disc? The drum brakes have more possibility of dragging. I know mine do.

[Reddeye]

100% sure it's the front. Not only are they hard to turn by hand with the wheels off, but when I've driven it and it happens the front brakes will smoke.

[garyd1961]

If you open the bleeder will fluid squirt out? If it does and the brakes release there is a hydraulic problem. It's the master cylinder, proportional value, or lines not releasing pressure. If no fluid squirts out and the brakes don't release it's a mechanical problem with the pads, the slides, or the piston in the caliper sticking.

[68 P.O.S.]

I've had that happen on one front wheel of my 72 before. When it would stick, it would smoke and eventually catch fire. It turned out that caliper needed a rebuild because the piston kept sticking in its bore.

If you check the master cylinder push-rod length, this is the tool you need: https://www.jegs.com/i/JEGS/555/631315/10002/-1

[Reddeye]

Today I jacked up the rear end and verified the wheels do move freely, while the fronts are still dragging. Leads me to a couple questions:
- Could this be because the proportioning valve doesn't push towards the rear as hard as the front?
- Would it be worth buying a new proportioning valve? Are they known to go bad?
- I plan on buying a new master cylinder anyway since mine is old. Will the pushrod length on a brand-new part be correct?

[BigBird05]

Discconect the brake hoses from the calipers and see if the wheels spin. You could have bad or warn out calipers.

[Reddeye]

I just swapped my rubber hoses for stainless ones last weekend and had it apart, and the calipers do release. They're also brand new.

[Steeveedee]

As a temporary measure, loosen the master from the boosted and slide in some 1/8" shims, and tighten it all back up. See if that helps.

[Richard2112]

Moisture in your lines and master cylinder (brake fluid is hygroscopic)can cause this to happen also. If you've got enough moisture in the system to cause this then you probably have some significant pitting in the bore of your master cylinder and your calipers.

[truck-kid]

I would suggest since you are going to replace the master cylinder anyway i would do that before replacing any thing else.Just be sure to check pushrod adjustment upon stall and report back what happens.

[pjmoreland]

I bought a used original proportioning valve from a 71-72 truck and was planning on using it for a disc brake conversion on my 68. I took the proportioning valve apart and found the internal components were seized from rust. It is safe to say proportioning valves can fail.

[Accelo]

I'm with PJMoreland.
" case of dragging brakes once, and it was because I didn't adjust the rod properly that pushes on the back of the master cylinder. As a result, the master cylinder piston wasn't able to retract 100% when I wasn't pressing on the brake pedal. I believe this traps the fluid pressure in the lines, preventing the brakes from releasing completely."
Check the rod, just shorten it a turn or so. It has to be close to the correct adjustment. You have done everything else. Time to eliminate it as a possibility.
Rick

[Asshat]

Not trying to hijack, but if I have drums all the way around, what would cause mine to stick. They don’tdo it when it’s on jack stands.

[Reddeye]

I ended up ordering a new master cylinder and booster assembly, I'll report back once I have it installed. I'll check out the old ones and see if I can find what the issue was.

[Reddeye]

Got the new master and booster installed, definitely made a difference. Fronts are SLIGHTLY dragging, but not enough to stop the truck from moving forward on a flat surface when in gear and no throttle (it wouldn't move before), and the fronts spin freely when jacked up and have the rotational mass of the tires and wheels. I'm still considering a new proportional valve though, just for peace of mind that the entire brake system is new.