GMC T-Series 1999 Owner's Manual page 102

Air Pressure Gage (If Equipped)
If the needle on the gage indicates that the air pressure is
below 60 psi (415 kPa), the PRIMARY BRAKE
warning light will come on. An alarm will also sound if
this happens.
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If your vehicle has air
brakes, this gage shows the
air pressure for both your
front and rear
brake systems.
The top pointer shows the pressure available for the
front system, while the bottom pointer shows pressure
for the rear. There should be no more than 4 psi (28 kPa)
difference showing between the systems.
Don't drive until both pointers are showing at least
120 psi (830 kPa), so you'll have enough air if you need
to apply your brakes.
Hourmeter Gage (If Equipped)
If your vehicle has an hourmeter gage, it will be to the
lower left of the steering column, mounted on a bracket
either above or below the fuse block. It shows the total
engine operating time.
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