This "Combo2" includes the new larger Opel GT 9-Inch power brake booster #4037-9, plus a new original-style 20mm GT master cylinder #4013. Fits your classic Opel GT at a special discounted price, with no core exchange required! Notes: Booster includes one way check valve in vacuum inlet nipple, and installation using a new booster hose part# 4041 is recommended. In some Opel GT's equipped with factory Air Conditioning, modification or adaptation is required for condensor to clear the booster.