Without the GMC brake booster working properly on your vehicle, the power assistance it provides to the braking system will no longer be available; impairing the stopping potential of you truck or van. The brake booster on your GMC truck is the component the brake master cylinder is mounted too. The power for this device is supplied by the vacuum the engine produces when it is running. The amount of vacuum being supplied to this braking component can be tested by pulling the large hose that is attached to the check valve in front of the brake booster when the engine is running. If vacuum is being supplied to this component is not assisting in the depression of the brake pedal, then the GMC brake booster should be the suspect malfunctioning part. The replacement of the brake booster on a GMC truck is an easy task to accomplish. The only component that has to be removed is the brake master cylinder. As long as the brake lines are kept attached to the master cylinder upon its removal, the system will not be breached and it will not have to be bled during the reassembling process. The brake booster is one of the components that cannot be rebuilt or bypassed by the owner of the GMC truck when it fails. The only solution is replacement it with a new or rebuilt part that has to be obtained from an auto parts store. Until this repair job is scheduled and completed, the braking system on the vehicle will be harder to use than the older style manual braking systems used in the past. Most drivers will find it difficult if not impossible to bring a moving truck to a stop without this power assistance working properly.