GMC Envoy 2003 Owner's Manual page 160

If the warning light comes on, there could be a brake
problem. Have your brake system inspected right away.
United States
This light should come on briefly when you turn the
ignition key to RUN. If it doesn't come on then, have it
fixed so it will be ready to warn you if there's a
problem.
3-40
If the light comes 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 the light is still on,
have the vehicle towed for service. See Towing
Your Vehicle on page 4-45 .
Canada
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CAUTION:
Your brake system may not be working
properly if the brake system warning light is
on. Driving with the brake system warning light
on can lead to an accident. If the light is still
on after you've pulled off the road and stopped
carefully, have the vehicle towed for service.