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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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