GMC T-Series 2007, Owner'S Manual, Page: 84 / 356

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.
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CAUTION:
If you apply any one of the air brake
parking controls while the vehicle is
moving, your 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 primary brake light and
warning 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
your 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 182.
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