Abstract: Automobile radiator cap assembly particularly suited to automobiles with coolant recovery systems in which coolant is blown through a venting duct in radiator filler neck. The cap assembly comprises a cap to fit over the upper end of the filler neck, a spring loaded primary element to seal against a ledge at the bottom of the filler neck, and a secondary sealing element having a cylindrical skirt portion extending internally within the filler neck and an annular flange portion projecting between the rim of the filler neck and the cap. A circumferential bead on the skirt engages the wall of the filler neck to provide a circumferential seal within the filler neck above the vent passage and the annular flange is clamped against the rim of the filler neck to provide a further seal.