Abstract: A monolithic radiator cap comprising a minimal number of inexpensive constituent parts engages the filler neck of an automotive cooling system. A valving gasket is normally held against the sealing surface of the filler neck by a spring which is responsive to thermal expansion of the cooling medium for controlled fluid flow out of the system. A seal is carried by the cap to engage the interior wall of the filler neck and hermetically seal the filler neck. The seal is maintained during the working and vent positions of the cap. Alternately, the cap includes auxiliary valving means for controlling fluid flow into the system in response to thermal contraction of the cooling medium.