GMC C series 2002 Owner's Manual page 108

When this light comes on, you will also hear a
warning tone.
Both of these lights should come on briefly every time
you start the engine. If they don't come on then, have
them fixed so they will be ready to warn you if there's a
problem. If one or both of these warning lights stay on
after you start the engine, there could be a brake
problem. Have your brake system inspected right away.
If the AUX BRAKE
warning light comes on, it
means that there is
something wrong in the
auxiliary brake system.
If one or both of these lights come on and stay on while
you are driving, pull off the road and stop carefully. You
may notice that the pedal is harder to push. Or, the pedal
may go closer to the floor. It may take longer to stop.
If either light is still on, have the vehicle towed for
service. See "Towing Your Vehicle" in the Index.
See "Hydraulic Brake Systems" in the Index for
more information.
CAUTION:
Your brake system may not be working properly
if a brake system warning light is on. Driving
with a brake system warning light 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.
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