Rear Brakes Locking up - HELP

[Marcos A]

Well I got me another OBS 96 Crew Cab this go around. I found this one locally, pretty clean and a good price so I couldn't pass it up. The previous owner did a 6 Lug Swap on it and a Low Mileage 97 motor/trans swap.

The issue that I am having is that whenever I apply the brakes (more than moderately) the rear brakes Lock up (Not all the time though, just if i apply firmer pressure). I can smash on the brakes and I can engage the ABS to the front wheels (because I can feel the pulsating Braking on the front wheels) but the Rear brakes lock up. I have taken and adjusted the adjuster star doohickey in the rear drums and it kinda helped a lil to loosen them up some but they still are locking up and I just don't know what to do now.

Any and All help is appreciated.

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[CKVortec]

have you taken the drums off to inspect them?
do you have oil or brake fluid all over the shoes?
are you missing any return or hold down springs?

 

[GMC Josh]

Make sure to look inside the rubber boots on wheel cylinder. Had that problem on my 71 and everything looked dry. I pulled the boot away from the wheel cylinder and quite a bit of brake fluid came out.

 

[TacosnBeer]

I had the same problem with my square body... check the backing plate behind the shoes. They will develop a groove and the shoe will stick on it and lock. I filed mine smooth and never had the problem again.

 

[Chevy-SS]

All good suggestions so far. Interestingly, if you have an axle seal leaking onto the brakes, it will cause them to lock up. You wouldn't think 90w gear oil would ever, in a million years, cause that..... but it does. Leaky wheel cylinder (already suggested) could also be doing it.

Anyway, time to pull apart the rear brakes. It's actually (and usually) a fairly easy job. The problem should be obvious. Jack up the truck and try rotating each wheel by hand, before taking apart, just to see if one (or both) are too tight. On drum brakes, you want just the tiniest bit of drag, just a tick of drag. I typically adjust them after completion, I adjust them until wheel locks up, then back off the star adjuster until wheel spins easy. This assures shoes have seated to the drums, and now have tiny tick of clearance.

 

[Marcos A]

Thanks Guys for all the suggestions. I guess I should have mentioned that I just had the Rear Diff Rebuilt (new Gears, Seals etc) but they did not touch the brakes (and the Brakes were doing this before the rebuild). so me and my buddy did exactly that we jacked the truck up and rotated the wheels then took the drums off and checked everything out. The Drums looked good and I had plenty of meats on the Pads (which were dry, the entire drum brake assembly was dry with no sign of any leaks). So we put it all back together and did exactly what Chevy SS said adjust until lockup then readjust until wheel spins easy - which again helped "some" but they are still locking up.

I did not look inside the wheel cylinders to see if they were leaking, nor looked at the backing plate for grooves. I guess I'll take them all off again and check all that out next. I might just replace the cylinders and Brake Hardware while I am in there just for piece of mind. Thanks Again!

 

[deadbeat]

I am not exactly sure which brake setup he has, but are the shoes on backwards? The long shoe should be in the rear if my memory is correct. Does the parking brake work? If so, does it work freely and do you use it?

 

[Joe Dirte]

Thanks Guys for all the suggestions. I guess I should have mentioned that I just had the Rear Diff Rebuilt (new Gears, Seals etc) but they did not touch the brakes (and the Brakes were doing this before the rebuild). so me and my buddy did exactly that we jacked the truck up and rotated the wheels then took the drums off and checked everything out. The Drums looked good and I had plenty of meats on the Pads (which were dry, the entire drum brake assembly was dry with no sign of any leaks). So we put it all back together and did exactly what Chevy SS said adjust until lockup then readjust until wheel spins easy - which again helped "some" but they are still locking up.

I did not look inside the wheel cylinders to see if they were leaking, nor looked at the backing plate for grooves. I guess I'll take them all off again and check all that out next. I might just replace the cylinders and Brake Hardware while I am in there just for piece of mind. Thanks Again!

If you havent pulled it apart yet try pullin ABS fuse. Have a 2000 esky that was randomly lockin brakes. Turned out the issue for me is in the abs system either the brain or one of the sensors. Havent gotten much further on it yet.

 

[Frank Enstein]

I recommend trying a different tack. The rears are working so let's try working on the front
.
First take the calipers off so you can remove the pads. Bolt the calipers on and see if you can slide the calipers in and out with 1 finger (you may need to push FIRMLY with 1 finger). If they don't slide freely clean and lubricate the caliper pins so they do.

Inspect the rotor friction surface as well as the pad, if they are glazed it will reduce the brakes effectiveness dramatically. The glaze can be removed with 120 grit sandpaper on both the rotors and pads. You only need to make them not shiny. Or even get the rotors turned/replaced and new pads.

Also make sure the caliper piston can be pushed back into the caliper without too much trouble. Rebuild/replace if the pistons are hard to push in not with 1 finger but maybe 1 finger on a c clamp.

Finally bleed the calipers properly (the pump 3 times and hold the pedal down with both feet and bend the steering wheel back method is not the right way. It simply wastes time and brake fluid) any air in the system will ruin the brake bias.

When you bleed brakes you want to move a lot of fluid but at low pressure. I like the Earl's Solo Bleed bleed screws they are a bleed screw with a built in check valve and make bleeding properly easy. Even by yourself.