GMC C series 2002 Owner's Manual page 184

Releasing Air-Operated Parking Brakes
(If Equipped)
If your vehicle has air brakes, you could have a special
towing problem. If your vehicle has to be towed
because of a complete loss of air pressure from both
systems, the parking brakes will have automatically
engaged. The tow operator can release the brakes
manually by using the following steps. Then your
vehicle can be towed with all wheels or just the rear
wheels on the ground.
1. Block the wheels of the vehicle.
2. Remove the release stud and nut from the side of the
brake chamber. Some chambers have studs which are
visible at all times.
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