Had a problem with rear brakes locking up. Only had 45K on the truck, but figured maybe 35 year old brake shoes weren't the hot setup Pulled the drums - piece o' cake - and had them turned; they'd never been off the truck before. Put in new shoes, hardware, and wheel cylinders. Worked great for awhile, and now they're locking up early again; feels like the drums have gone out of round.

The brakes aren't dragging. I don't overload the truck or brake heavily down mountain roads. I've never experienced this before, and I've been playing with cars since disc brakes were exotica.

 

Wheel bearing check by jacking the rear and giving top and bottom a good push and pull to check for loose bearings could eliminate that.

Also, how are the front brakes? If the rear is 100% and the fronts are weak that could throw it out of balance. I'd start with visual inspection and bleeding. Also rubber hoses with hidden blockage or swelling can cause weak brakes.

Beyond that and another visual of the rears, it could be the proportioning valve. They are available from inline tube if not from your local AP store.

 

What do you mean when you say they are "locking up early"?

 

I haven't checked the bearings. Thanks for that tip. Hoses are good. Fronts were done at the same time I did the rears. Put in a new master, too, and flushed/bled the whole system. When I had the lockup problem the first time, I thought it was the proportioning valve. One of the guys on the forum (can't remember who... sorry!) told me to check for leaky wheel cylinders; contaminated brake shoes would make the brakes grabby. Sure enough, shoes were wet. Drums were slightly out of round, so had 'em turned. New brakes worked great for about a year, and now the rears are locking up prematurely again.

When I brake hard, the rears lock, which is not conducive to stopping in a straight line Also gets a lot of unwanted attention.

 

I would jack up the rear end. Adjust the brakes so that they sightly drag, then see what happens.

 

Did the bearing check. There's a slight amount of movement; same on both sides. You can feel it, but you can't see it, so I'm guessing it's probably OK. Found the right adjuster hadn't been working. Pulled it apart, cleaned it up, and adjusted both brakes for a very light drag.

Rears still lock up too early, in my opinion. I may re-do the fronts with some different pads - currently using Wagner Thermo-Quiets - and, if that doesn't do the trick, look into a new proportioning valve.