GMC 2003 C4C042 Owner's Manual page 117

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, hold the regular brake pedal down
to keep your rig from moving.
2-42
Pulling it out applies the parking brake. The park brake
light will come on when the air parking brake is applied.
If your vehicle has air
brakes, you will have this
parking brake. It is located
above the radio in the
instrument panel.