GMC 2003 C4C042 Owner's Manual page 157

Air Brake System Warning Light
Vehicles with air brake systems have one brake system
warning light on the instrument panel.
United States
The LOW AIR warning light should come on, as a
check, whenever you start the engine. However, it is
designed to come on, and stay on, when brake reservoir
pressure has dropped below 60 psi (410 kPa).
2-82
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:
If the LOW AIR warning light comes on and the
warning tone sounds, the vehicle can stop
suddenly and without further warning. This is
Canada
because the parking brake will come on if the
pressure falls below 35 to 45 psi (240 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.
CAUTION: