It is the Chevrolet brake booster that allows for that small amount of force applied through the depression of the brake pedal to be converted to a large amount of pressure to be applied to the brake master cylinder. The brake booster is the large round device the master cylinder is mounted to under the hood of your Chevrolet. The brake booster on your Chevrolet assists in helping to move the piston in the master cylinder to compress the hydraulic brake fluid in the system. The exact amount of assistance can be noticed by applying the brake when the car is not running since the brake booster will not be functioning. Compressing hydraulic fluid requires a great amount of force and without the booster, it is almost impossible to stop a car. The Chevrolet brake booster is not broken when it is not working with the engine not running. This auto part works off of the vacuum your engine generates. Without this vacuum or a reduced amount of vacuum, the assistance in helping compress the hydraulic fluid is reduced. This reduction can be a leak in the vacuum system of the brake booster itself or in the amount that is being supplied to this part from the engine. Your technician should test the amount of vacuum this part is receiving before replacing this component on your car. If this part does fail, it is due to a failure of a seal on the inside of it which your technician cannot replace, the entire part has to be replaced. You will notice there is a problem when you attempt to stop your Chevrolet while traveling down the road and the amount of force you have to apply to the brake pedal is greater than it had been. Nothing else in your brake system has this symptom.