GMC 2010 T-Series Owner's Manual page 64

To release the parking brake, first push down the
regular brake pedal. Then, while pressing in the release
button, push the parking brake lever all the way down
and shift the transmission to N (Neutral). The parking
brake light will stay on if the parking brake is not fully
released.
Notice: Driving with the parking brake on can
overheat the brake system and cause premature
wear or damage to brake system parts. Verify that
the parking brake is fully released and the brake
warning light is off before driving.
Parking Brake (With Air Brakes)
Pulling it out applies the parking brake. The parking
brake light will come on when the air parking brake is
applied.
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If the vehicle has air
brakes, you will have this
parking brake. It is located
above the radio on the
instrument panel.
Vehicles built for use as tractors or towing vehicles have
two air brake controls. They look like this:
WARNING:
When the yellow PARKING BRAKE and the red
TRAILER AIR SUPPLY knobs are both pushed in,
the 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 the rig from moving.