GMC 2005 T-Series Owner's Manual page 102

Air Pressure Gage
If the needle on the gage indicates that the air pressure
is below 60 psi (415 kPa), the primary brake light will
come on. An alarm will also sound if this happens.
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.
3-38
If your vehicle has air
brakes, this gage shows
the air pressure for
both your front and rear
brake systems.
Hourmeter Gage
To access the engine hourmeter gage press the trip
odometer button three times.
The instrument panel cluster displays the accumulated
engine run time hours using the reconfigurable odometer
display. The engine run time hours displays only when
the ignition is in the LOCK or ACC (Accessory)
position and the trip reset switch is pressed for about
four seconds.
When the ignition is in the LOCK or ACC (Accessory)
position, the instrument panel cluster displays the engine
accumulated hours for up to 30 seconds. The value of
the accumulated hours does not change when the
battery is disconnected.
The hourmeter resets to 0.0 when 10,000 hours are
accumulated. The instrument panel cluster begins
to reaccumulate the engine run time hours from
0.0 hours.