GMC T-Series 1999 Owner's Manual page 66

If you have the TRACTOR PARKING (blue) knob, pull
it out to set the tractor brakes while the air system
is charging.
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) and you may hear
the "pop off" valve operate.
For driving with a trailer, the yellow, red and blue (if
equipped) knobs all must be pushed in. It's the same
when you're not pulling a trailer except that the
TRAILER AIR SUPPLY (red) knob must be 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.
2-30
If the air pressure drops below 60 to 70 psi
(413 to 482 kPa), the PRIMARY BRAKE light will
come on. If the air pressure drops to 40 to 45 psi
(275 to 309 kPa), the yellow knob will automatically
pop out and apply the spring brakes. 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" in the Index.
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.
yellowblue