front disc brakes keep locking up!!!
 

Ok, six months ago, I had the master cylinder replaced and the front disc pads also replaced. Two weeks ago, I was driving down the road and I noticed it was getting harder and harder to go. So, turns out, that the front brakes on both sides were locked up.
I released them by pulling out the two main lines running into the master cylinder and quickly replaced them.

I had the plunger rod adjusted from the linkage to the master cylinder by a mechanic. It was fine for about two weeks, then they locked up again. During this time I noticed a lot of differentiation in pedal pressure...sometimes touchy, sometimes normal. Now the pads squeak really bad. By pulling off the main lines from the master cylinder releases the pads, albeit temporarily, but what could it be? The power brake booster? Is the pedal switch nut out of adjustment? Could the calipers be all out of whack? I don't know....:confused: Can anyone help???

I forgot to mention that the truck is a 1972 longbed with front disc and rear drum brakes. Thanks!

I'm not real sharp on diagnosing brake problems, but it sounds like your brake booster isn't releasing the master cylinder all the way.

Do you have power brakes?

If so. there is linkage on the back side of the booster that can get rusty and make the pedal weird. If you take your favorite version of wd40 and spray the heck out of the linkage and keep working the pedal; it might take care of the problem.

Steve

:flag:

In disk brake systems there is a residual pressure valve so that the brake pads dont retract all the way each time the pedal is released. They only retract far enough to get them off the surface of the disk. Theyre sometimes located different places in different vahicles but the residual pressure valve is probably located in the proportioning valve block located under your master cylinder. It may be sticking and in need of replacement.

68speedalert,

I do have power brakes...I also looked at the linkage behind the booster yesterday and I did notice rust on the rod, so maybe I'll blast it with PB Blaster and pump the brakes to see if that helps. Thanks!

Mikep,

I ordered the porportioning valve block and a new power brake booster from LMC truck yesterday, so hopefully that will also solve the problem. Thanks for your help!

Is it adviseable to drive it regularly for a couple days before I can get the parts and fix the problem?

I dont think I'd do it. Ive blown out tires on the hiway before from driving with sticking brakes. The least it will do is wear your rotors excessively. It could also cook out the grease from the hub bearings and boil your brake fuid. If you dont mind bleading your brakes you might want to try taking the proportioning block off and disassembling it and cleaning everything up with fine emery cloth or OOO stel wool. Not much damage you can do as you will be replacing it in a few days but if theres some burrs or more likely rusty areas that are causing the residual pressure valve to hang up then you may get a few more days ( or more ) of safe driving out of it .

BTW ... Get a different mechanic. That was a no brainer for a decent brake tech.

I had the same problem and it was a defective master cylinder I had just replaced. Replaced it again [with one I bought somewhere else] and no more problems