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Below is a collection of Jim Grant's Tech Tips sorted by Vehicle Make. These Tech Tips were answers to questions submitted to Jim by users over the course of many years.
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  Jim Grant's Tech Tips

'86 GMC C3500, Brake Lock Up

Q: The passenger side rear wheel on my ‘86 GMC C3500 truck locks up under light pedal pressure. The driver’s side wheel grabs hard but will not lock up. It is worse the first few miles of driving but will still lock up on gravel roads after driving all day. I have replaced the shoes and brake return springs, but still have the problem. This sure makes driving in the snow quite exciting. Any ideas you might have would be appreciated as winter is just around the corner.

- Lee C.

A: There is a device in the brake line to the rear brakes on your vehicle, that is called a proportioning valve. The job of the proportioning valve is to slow/reduce brake fluid pressure to the rear wheels. With age, it is not uncommon for the seals/o-rings in this valve to swell or for corrosion to cause the valve to stick. This valve can stick in about any position and cause a reduction or increase in brake fluid pressure to the rear brakes. Your truck may have also been equipped with a load-sensing valve that was connected to the rear end of the vehicle to the body of the vehicle. A heavy load causes the body of the vehicle to drop. The load-sensing valve, in response, allows greater brake fluid pressure to the rear brakes to handle the heavier loads when stopping. It is not uncommon to find the load sensing valve linkage from the body of the truck disconnected or broken. Also , because of your vehicle’s age I would question the inside diameter of the brake drums. An over sized brake drum from too much machining or wear will cause an imbalance in braking. For example, we had a vehicle that had no problem with locking-up the right rear brake. The brakes looked great on this vehicle. Swapping the brake drums from side to side found that the problem moved to the other side. The fix was a new pair of brake drums. In some cases, you have to look beyond the brake shoes and hardware for a braking problem.

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