An apparatus (30) for adding coolant (64) to a cooling system of a motor vehicle includes a cap (32) with a resilient sleeve (82) that expands against the inside wall of a radiator filler neck (24) to provide an airtight connection. A valve (36) attached to the cap (32) controls the flow of air and coolant through the cap (32). A gauge (34) on the cap (32) indicates the pressure inside the radiator (20). A venturi assembly (44) connected to the valve (36) provides a source of vacuum (40) for evacuating air from the cooling system. Thereafter, coolant (64) is drawn through the cap (32) by the vacuum created in the system.