Disc brakes locking up randomly 76 stock

[Tito]

Just recently the front d44 stock disc setup started locking up while driving down the road. It has a power vacuum Booster. I've got it all apart and am looking at everything currently, there seems to be a lot of potential sources for the issue so I was hitting up the hoard to hear their experiences to save me some potential time, I don't have the money to start throwing at it like it's on a pole... Even if she's pink.

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[Apogee]

I had similar issues...I had residual pressure at both front calipers, which could be released if I cracked either bleed screw. Apparently the booster was failing and not allowing the master cylinder to fully return against the snap ring, which was maintaining the residual pressure on the calipers. It would start out fine, but after a mile or two, the brakes were smoking and it was everything I could do to maintain forward momentum...although 4WD certainly helps to drive through the brakes in a pinch if you need to.

Upon further inspection, the MC was failing, and slowly leaking brake fluid into the booster, so no real surprise there.

I took the opportunity to go through the rest of the system, and even though everything checked out, I did put some new calipers on it since they were ~24 years old and some fresh pads since they were glazed and had seen better days.

The only other possible causes I can think of would be an issue with the front center hose, causing it to maintain residual pressure, or the combination valve. By cracking the system when locked up in various places, you should be able to figure out which it might be, but I'd look at the MC first just cause that's the most likely thing to fail.

 

[B RON CO]

Hi, did the problem just start out of nowhere, or did you do some work on it first.
I would check that a caliper is not stuck, meaning the piston easily pushes into the bore.
Also make sure the caliper slides are clean and lubed.
Good luck

 

[blubuckaroo]

Have you recently replaced the master cylinder?
A drum brake master cylinder has a residual pressure check valve that will do that if used on disc brakes.

 

[Broncobowsher]

Check the length of the pushrod coming out of the booster into the master. If it is adjusted too long you get exactly what you describe. Too short and you get excessive pedal travel. The quick check, the master should mate cleanly to the booster and not be drawn up to it by tightening the nuts.

 

[Rustytruck]

How clean was the brake fluid when the calipers were sticking? how often did you drive this truck?

 

[Tito]

Thanks guys, diving in right now. The MC, calipers, rotors, hoses were all brand new when I did frame off 12 years ago. It probably has 4k miles on it in that time. Holy crap... I can't believe that was 12 years ago. Everything replaced was stock, no upgrades or mods on this one. Fluid is pretty black, I'm going to flush the system and see what I've got going on. I've got a replacement MC to fiddle with as well.

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[DirtDonk]

If you get to the point of taking the calipers off, make sure the sliding surfaces are clean, smooth and lubricated with brake grease or anti-seize. Keep the rust at bay for the future.

And 12-year old calipers can stick. But so can other stuff, so good luck with the detective work.

Paul

 

[dsenatore]

Tito,
I had the same issue on my '76. I'd have to pull over while driving and open the bleeder screw to relieve pressure. Turned out it was the rubber brake lines going to the calipers. They would bulge when I applied the brakes and maintain pressure when brakes were released. Put on new lines and problem stopped. It was a while ago but I think I had someone apply the brakes while I held and watched the brake line and it was evident what was going on. The brake lines were not that old so I think they were defective. Hope it's that easy for you.
David

 

[Tito]

Well I started with flushing the brake fluid through, especially the front then took the master cylinder off. There was some corrosion on the plunger so I cleaned that up and put the new master cylinder on. So far put about 40 miles on it and all good now. Thank you peeps! She's ready for some beautiful spring weather now.

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