GMC 2005 T-Series Owner's Manual page 92

When this light comes on, you will also hear a warning
tone. Have the system repaired immediately.
Both of these lights should come on briefly every time you
start your 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 one or both of these lights come 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 BRAKE light is still on, have the vehicle towed for
service. See Towing Your Vehicle on page 4-32. See
"Hydraulic Brake Systems" under Braking on page 4-5
for further information.
3-28
If the service brakes soon
warning light comes on, it
means there may be
something wrong in the
brake system.
Parking Brake Warning Light
This light will also come on when you set the parking
brake. The light will stay on if your parking brake
does not release fully.
Air Brake System Warning Light
Vehicles with air brake systems have a brake system
warning light on the instrument panel.
United States
The LOW AIR warning light should come on, as a
check, whenever you start the engine. However, it is
designed to come on, and stay on, when brake reservoir
pressure has dropped below 60 psi (410 kPa).
Canada