GMC C series 2002 Owner's Manual page 109

Air Brake System Warning Light
Vehicles with air brake systems have one brake system
warning light on the instrument panel.
However, it is designed to come on, and stay on,
when brake reservoir pressure has dropped below
60 to 70 psi (410 to 480 kPa).
When this warning light comes on, you will also hear a
warning tone. If the warning light and tone come on
while you are driving, you should drive only as far as
the nearest point of safety and then stop the vehicle.
Here's a very important thing for you to know:
2-66
The LOW AIR warning
light should come on, as a
check, whenever you start
the engine.
CAUTION:
If the LOW AIR warning light comes on and the
warning tone sounds, the vehicle can stop
suddenly and without further warning. This is
because the parking brake will come on if the
pressure falls below 40 to 45 psi (275 to 310 kPa).
You or others could be injured. If the LOW AIR
warning light comes on and the warning tone
sounds, stop as soon as you can. You won't
know how quickly the system is losing pressure,
so be aware that the parking brake may come
on suddenly.