GMC 2009 Yukon XL Owner's Manual page 225

If the warning light comes on and a chime sounds there
could be a brake problem. Have the brake system
inspected right away.
This light can also come on due to low brake fluid.
See Brakes on page 5-40 for more information.
United States
This light comes on briefly when the ignition key is
turned to ON/RUN. If it does not come on then, have it
fixed so it is ready to warn if there is a problem.
If the light comes on while driving, pull off the road and
stop carefully. The pedal might be harder to push or
Canada
might go closer to the floor. It can take longer to stop.
If the light is still on, have the vehicle towed for service.
See Towing Your Vehicle on page 4-44.
CAUTION:
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The brake system might not be working properly if
the brake system warning light is on. Driving with
the brake system warning light on can lead to a
crash. If the light is still on after the vehicle has
been pulled off the road and carefully stopped,
have the vehicle towed for service.
3-41