GM Service Manual Online
For 1990-2009 cars only

Your vehicle has a panic brake assist system that monitors the intention of the driver while braking. If the system senses that the driver has applied hard/fast pressure to the brake pedal, the system will generate additional pressure, making it easier for the driver to maintain brake application. When this happens the brake pedal will feel easier to push. Just hold the brake pedal down firmly and let the system work for you. You may feel the brakes vibrate, or you may notice some noise but this is normal. The brakes will return to normal operation after the brake pedal has been released.

The brake assist system becomes operational after the vehicle has accelerated to a speed in excess of approximately 6 mph (10 km/h). It stops operating when the vehicle decelerates to a speed below approximately 3 mph (5 km/h).