Chevrolet T-Series Owner's Manual page 108

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.
However, it is designed to come on, and stay on,
when brake reservoir pressure has dropped
below 60 psi (410 kPa).
108
The LOW AIR warning
light should come on, as
a check, whenever
you start the engine.
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:
CAUTION:
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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.