GMC 2010 T-Series Owner's Manual page 145

After Towing
1. Block the rear wheels and release the steering
wheel.
2. Apply the parking brake system before
disconnecting from the towing vehicle.
3. Check and fill the rear axle with oil as required.
Releasing Air-Operated Parking Brakes
If the vehicle has air brakes, there could be a special
towing problem. If the 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 the vehicle can be
towed with all wheels or only 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.
4-23