Chevrolet kodiak 2004 Owner's Manual page 104

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).
For driving with a trailer, the yellow and red (if equipped)
knobs must be pushed in. It's the same when you're
not pulling a trailer except that the red trailer air supply
knob must be pulled out.
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. Don't
apply any one of these controls while you're
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.
2-36
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 4-31.
CAUTION:
{
If your vehicle is left in gear, the engine can
start if the vehicle starts to roll. This can easily
happen if you have the diesel engine. Shift the
transmission into NEUTRAL (N) before you
leave the vehicle.