Extension piece in translucent white plastic screws on metal canister type master cylinders only. The seal provided slips over the thread of the filler neck and sits on the tank for the extension piece to screw down onto and seal. The original cap fits the top giving you a clear idea how much fluid is in your tank and extension piece without removing the cap, also effectively increasing fluid capacity. As your pads wear down or under hard cornering it gives you a peace of mind and insurance that you have enough fluid. Originally fitted by Special Tuning and used on MG's and other early type single line braking systems this will not fit plastic tank type master cylinders as the rubber seal cannot sit squarely on the tank to seal.