GMC 2010 T-Series Owner's Manual page 103

Parking Brake Warning Light
This light will also come on when you set the parking
brake. The light will stay on if your parking brake does
not release fully.
Air Brake System Warning Light
Vehicles with air brake systems have a 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).
When the warning light comes on while you are driving,
you should drive only as far as the nearest point of
safety and then stop the vehicle.
Canada
{
WARNING:
If the air brake system 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 35 to 45 psi (240 to 310 kPa).
You or others could be injured. If the air brake
system warning light comes on and the warning
tone sounds, stop as soon as you can. You will
not know how quickly the system is losing
pressure, so be aware that the parking brake
may come on suddenly.
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