GMC C-Series 2001 Owner's Manual page 76

CAUTION:
If you apply any one of the air brake parking
controls while the vehicle is moving, your rig will
stop suddenly. If you are not ready for this, you
or others could be injured. Don't apply any one
of these controls while you're driving, unless you
have to make an emergency stop.
If the air pressure drops below 60 to 70 psi (413 to
482 kPa), the PRIMARY BRAKE light will come on.
If the air pressure drops to 40 to 45 psi (275 to
309 kPa), the SYSTEM PARK knob will automatically
pop out and apply the spring brakes. If you ever have a
complete loss of air so that your air brakes automatically
apply, there is a way that the tow operator can release
the parking brakes to tow the vehicle. See "Towing Your
Vehicle" in the Index.
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CAUTION:
If your vehicle is left in gear, the engine can start
if the vehicle starts to roll. This can easily happen
if you have the diesel engine. Shift the
transmission into NEUTRAL (N) before you
leave the vehicle.