2003 GMC Sierra 1500 Pickup Owner's Manual - Page 219

REDUCED BRAKE POWER If your vehicle has vacuum assist power brakes, this message will be displayed and you may notice that the brake pedal is harder to push and it will take longer to stop. You may also hear a motor running and feel a slight vibration in the brake pedal or steering wheel when you apply or release the brake pedal even if your foot is not on the brake pedal. This indicates that the Supplemental Brake Assist system is working to maintain braking power. If you are braking lightly, you may not notice any difference in the operation of your brakes. This message may also be displayed for brief periods if you are driving at higher elevations and are pumping your brakes or braking hard. The message may clear after you've lifted your foot from the accelerator pedal an allowed the vehicle to coast in gear or after you have driven down to a lower elevation. This is a normal operation of your brake system and does not require that the brake system be serviced. However, if the message does not clear, your brake system needs service. While the message is displayed you will notice that the brake pedal is harder to push and it will take longer to stop. See Brake System Warning Light on page 3-40. Your brake system may not be working properly if theREDUCED BRAKE POWER message is displayed. If this message comes on and stays on while driving, pull off the road and stop carefully. The brake pedal will be harder to push andit will take longer to stop. If the message is no longer displayed after you have pulled off the road and stopped, you can continue driving. However, if the message is still displayed, or if it comes on again when you are driving, there is a problem with your brakesystem. If you continue to drive with this message displayed it can lead to an accident. You should have the vehicle towed for service. Also, see Brake System Warning Light in the Index. 3-64