GMC 2003 GMT560 T-Series Owner's Manual page 91

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).
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. Here's a very important
thing for you to know:
Canada
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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 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.
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