Low Vacuum at High Altitude - keywords 2.8L booster brake LP1 low vacuum

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Some customers may comment of hard brake pedal within the initial 60-90 seconds after a cold start, if the vehicle is at altitude greater than approximately 5000 ft above sea level. This condition would be only be noticed in very unusual situations where the brake pedal is depressed several times after the engine is started, and the throttle is not opened at all. In this specific condition available engine vacuum at idle RPM may be reduced.

Call General Motors Technical Assistance to create or update the case and receive further instructions.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.