GMC T-Series 1999, Owner'S Manual

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CAUTION:
Your brake system may not be working properly
if a brake system warning light is on. Driving
with either of the brake system warning lights on
can lead to an accident. If either light is still on
after you’ve pulled off the road and stopped
carefully, have the vehicle towed for service.
Air Brake System Warning Light
Vehicles with air brake systems have one brake system
warning light on the instrument panel.
The PRIMARY BRAKE
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 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:
CAUTION:
If the PRIMARY BRAKE 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 PRIMARY
BRAKE 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.