A vacuum reserve canister captures an extra shot of vacuum from your engine for your power brakes. It is for cars equipped with big cams and power brakes. This unit approximately doubles the volume of vacuum available for your power brakes. It is recommended for cars with 14 inches of vacuum or less. The COMP Cams® Vac+ is made of spun aluminum which eliminates the hazard of flaking rust particles contaminating the brake system and provides a 50% weight savings over the heavy steel cans of the past.