GMC 2010 T-Series Owner's Manual page 200

3. Using the handle located at the bottom of the cab
toward the back, lower the cab until the hook
catches and the cab is held down.
4. Push the primary tilt lever upward fully to lock the
cab to the chassis.
5-52
WARNING:
An unsecured cab could move suddenly causing
an accident or an injury. Be sure the tilt cab is
properly secured to the chassis before moving the
vehicle.
5. Reinstall the pin and
turn it to keep the
primary tilt lever from
moving downward.