Fast, economical and accurate method to test for combustion gas leaks from head gasket, cylinder head & engine block. Includes expansion tank adapter with vent allowing the end user to securely attach adapters to radiator or expansion tank when performing a combustion gas leak test. Quick connect coupler will attach to the radiator cap and expansion tank adapters in the Mastercool 43300, 43301, 43302, 43305 and 43306. The test cap adapter will provide a leak free secure connection to ensure the correct testing of the cooling system without the use of a rubber cone.