Abstract: A check valve (32) comprises a housing (33) having an inlet (34) and an outlet (40) defined therein and first (43) and second (45) ramp surfaces are defined in the housing (33) at varying acute angles (a,b) relative to a longitudinal axis (X) of the valve (32). A valve element (41) is mounted in the housing (33) for sequential movement along the ramp surfaces (43,45) to close the outlet (40) in response to fluid flow through the valve (32). The check valve (32) finds particular application for use as a vent valve in a cooling system (11) for an internal combustion engine to avoid the overcooling problem which is prevalent with conventional cooling systems wherein a continuously open port is employed in association with a thermostat for air-purging purposes.