GMC C series 2002 Owner's Manual page 73

Air Brake (If Equipped)
CAUTION:
When the yellow SYSTEM PARK and the red
TRAILER AIR SUPPLY knobs are both pushed
in, your rig will be free to move. It could strike
someone or something. When both of these knobs
are pushed in, either hold the regular brake
pedal down or, if you have it, pull out the blue
TRACTOR PARKING knob, to keep your rig
from moving.
2-30
If your vehicle has air
brakes, you will have this
control. Pulling it out
applies the parking brake.
The PARK BRAKE light
will come on when the air
parking brake is applied.