GMC 2006 Sierra - Manual (Page 220)

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 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 4-62.
{ 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
have pulled off the road and stopped carefully,
have the vehicle towed for service.
Anti-Lock Brake System
Warning Light
With the anti-lock brake
system, this light will come
on when you start your
engine and may stay on
for several seconds.
That’s normal. If the light doesn’t come on then, have it
fixed so it will be ready to warn you if there is a problem.
If the light stays on, or comes on when you’re driving,
your vehicle needs service. You will also hear a chime
sound when the light is on steady. If the regular brake
system warning light isn’t on, you still have brakes, but
you don’t have anti-lock brakes. If the regular brake
system warning light is also on you don’t have anti-lock
brakes and there’s a problem with your regular brakes. In
addition to both lights, you will also hear a chime sound
on the first occurrence of a problem and each time the
vehicle is shut off and then restarted. See Brake System
Warning Light on page 3-41.
3-42