GMC 2010 T-Series Owner's Manual page 65

To charge a trailer's air brake system do the following:
1. Move the 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 the 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 3 39 for more information.
For driving with a trailer, the yellow and red, if equipped,
knobs must be pushed in. When you are not pulling a
trailer, the red trailer air supply knob must be pulled out.
WARNING:
If you apply any one of the air brake parking
controls while the vehicle is moving, the rig will
stop suddenly. If you are not ready for this, you or
others could be injured. Do not apply any one of
these controls while you are driving, unless you
have to make an emergency stop.
If the air pressure drops below 60 to 70 psi
(413 to 482 kPa), the low air warning light and buzzer
will come on. If the air pressure drops to 35 to 45 psi
(241 to 310 kPa), the red trailer air supply knob will
automatically pop out and apply the spring brakes on
the trailer.
If the air pressure drops to 35 to 45 psi (241 to 310 kPa),
the yellow park brake knob will automatically pop out
and apply the spring brakes on the truck or tractor.
If you ever have a complete loss of air so that the 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 on page 4 20.
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