There is a reinforcement plate that attaches to the back of the booster. Then there is a foam gasket that goes between the reinforcement plate and the firewall. You need the gasket there to keep water and possibly fumes out of the interior. You can buy complete body gasket kits from DMT(Detroit Muscle Technologies) If you don't want to buy a whole kit, you could probably make a gasket yourself. If you plan on removing/replacing other components, the gasket kits will come in handy. There was also a gasket between the master cylinder and the booster.