GMC C-Series 2001 Owner's Manual page 183

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.
5-16
2. Remove the release stud and nut from the side of the
brake chamber. Some chambers have studs which are
visible at all times.