GMC 2010 Yukon/Yukon XL Owner's Manual page 258

With the ignition on, the brake system warning light
comes on when the parking brake is set. If the vehicle
is driven with the parking brake engaged, a chime
sounds when the vehicle speed is greater than
8 km/h (5 mph).
The vehicle's hydraulic brake system is divided into two
parts. If one part is not working, the other part can still
work and stop the vehicle. For good braking, both parts
need to be working well.
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 6 42 for more information.
4-36
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.
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.