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Old 01-22-2004, 07:03 AM
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Rear wheel lock up

I am having a problem with my 86 GMC 1-ton P/U. The passenger side wheel locks up with the slightest touch of the brake pedal. I have bled the brakes, installed and correctly adjusted new shoes, swapped drums, checked both wheel cylinders for leaks and still have the problem although it did get better for a few days after working on the shoes. This truck has a proportioning valve and load sensing valve. I am wondering if one or both of these items are junk and how could I tell which one? Any advice will be appreciated.

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Old 01-22-2004, 07:58 AM
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Arrow

Are you sure one or both wheel cylinders are not frozen and hanging on application/release? One of the steel brakes lines damaged? Flex hose in good condition?

How about return springs...the pads the shoes themselves slide on? Are they grooved preventing proper shoe travel/retraction? Did you lightly lubricate them?

If you think you have a bad valve, about the only sure way to isolate it is with brake pressure gauges. Otherwise you will be guessing.
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Old 01-22-2004, 02:30 PM
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Brake lines are new as well as all brake springs. Shoes and drums have no ridges. I don't own brake pressure guage so will probably have to go ahead and replace proportioning valve and hope for the best. Thanks for your help.
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Old 01-23-2004, 04:51 AM
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I'm not sure on the 1 ton, but, check to see if one shoe's pad is longer than the other, the short shoe should be forward, the longer on the rear. just a thought!
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Old 01-24-2004, 10:24 AM
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Thanks for the reply Wrencher but short shoes are forward on both sides. This problem is still giving me fits.
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Old 01-24-2004, 10:39 AM
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My 75 Blazer used to do the same thing. Ended up replacing both wheel cylinders and all rear brake lines. Solved the problem, maybe it will work for you.
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Old 01-25-2004, 11:35 PM
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It would seem that if it were either the load sensing valve or proportioning valve, then both wheels would lock up since both of these would send the same pressure to both wheals. Most things I can think of have already been mentioned. Have you checked for anything loose or broken on the grabbing side that would allow either shoe to shift and pull in to the drum? Only other thing I could think of is something in the shoe lining itself. Might be interesting if all else fails, to switch shoes from side to side and see if the problem follows the shoes.
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Old 01-27-2004, 03:56 PM
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Cylinder

Seems to me that Kultz asked about the Cylinder on that offending brake and you never really answered that you had looked at it. Can you back the adjustment out far enough that the wheel never stops? It is a manual adjustment and very easy to make. All valves and proportioning are done after they leave the master cylinder. Thus both brakes should be effected if one of them where the problem. My truck had similar problem and it was the wheel cylinder. I honed it and flushed the old fluid. Works fine now.

hr41pearl
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Old 01-27-2004, 04:17 PM
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Wheel cylinders did not show any signs of leaks when I put new shoes on. Maybe froze up on the inside? Would the bad cylinder be on the wheel that is locking up or opposite? I will try backing off the shoes on that side after I get front brake lines finished. I sure do appreciate everyones replies. This site is an amazing source of information. Thanks.
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Old 01-27-2004, 04:38 PM
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EITHER

Could be either one of them. Bad not letting anything happen. Sticky not letting the spring do its full job of return. They come in a kit of two. Best to clean and hone them both. If you are doing brakes, heck, just do them all. Cylinders cause more problems than any other part of the brake system. New lines are nice, but very rarely the problem. I flush the brake fluid about every 5 years to remove moisture. I have seen it get nasty.

hr41pearl
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Old 01-27-2004, 06:31 PM
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Would you recommend honing existing or replace with new? New were priced at about $10 each but probably made in a grass hut somewhere. Thanks.
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Old 01-27-2004, 06:54 PM
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You can rebuild the cyls your self if you have a hone. For the proce of new ones, I usually just replace them. With most rear drum brakes, you have metal lines going to each wheel from 1 common line coming back. If one wheel is grabbing, it could be that the other one is not working, forcing the pressure to the other one. Or the side that is grabbing could be not retracting, therefore only needs a little nudge to fully apply. I would replace both.
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Old 01-28-2004, 10:13 PM
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Muley
I had a 86 chevy van that did the same thing and done eveything you did come to find out the emergency brack cable was not releasing all the way right at the backing plate this was stopping the shoes from completely seating. You have to have the drum off to be able to see what I mean, where the cable hooks to the arm on the shoe needs to be pried back then put the drum and wheel back on without setting the park brake try a road test. If this fixes the problem you will have to replace both park brake cables at the drums.

Jim
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