Abstract: A hand-operated pump is combined with a closure cap for sealing and pressurizing the open space within a carbonated beverage container. A pump cylinder is integrally formed with a closure screw cap and is insertable through the neck of the beverage container. A piston is coupled by a retainer ring for extension and retraction through the pump cylinder. The piston carries a floating seal which is axially movable along a reduced diameter portion of the piston for opening and closing an air inlet port between an air supply annulus and a compression chamber. The pump includes an improved check valve assembly in which a compressed air discharge port is sealed by a resilient membrane which engages a tapered sealing surface which is coincident with the compressed air discharge port.