The original "Combo" includes Opel GT Source's exclusive new, larger 9-inch power brake booster #4037-9, plus a new "Big Bore" 22mm GT Master Cylinder #4013-22. This popular upgrade gives your GT a modern brake pedal response. All new, this combination is offered at a special discount, with no core exchange required! Notes: Booster incorporates the one way check valve in the vacuum inlet nipple, and we recommend installation using a new booster hose (part #4041). Note: May require modification or adaptation to fit in Opel GT's equipped with factory Air Conditioning.