GMC T-Series 2007 Owner's Manual page 83

Vehicles built for use as tractors or towing
vehicles have two air brake controls. They look
like this:
CAUTION:
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When the yellow PARKING BRAKE 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.
To charge a trailer's air brake system do the
following:
1. Move your vehicle into the proper position.
2. Apply the parking brake by pulling the yellow
parking brake knob out.
3. Hook up the trailer air system properly.
4. Get into the vehicle.
5. Push and hold down the regular brake pedal.
6. Push in both the yellow parking brake and
the red trailer air supply knobs. This will
charge your trailer's air system.
After a few minutes, the trailer system should be
fully charged. When it is, the air pressure gage will
show about 125 to 135 psi (862 to 931 kPa).
See Air Pressure Gage on page 136 for more
information.
83