A spout for dispensing liquid from a closed container has a cap that fits over a fitment or neck on the container, the cap being provided with a sleeve within which a reciprocating barrel may slide from a closed position to an open position. The barrel has a longitudinal vent groove on its upper surface that serves as a passage to admit atmospheric air to equalize internal and external pressure. The cap is provided with a flap that blocks the groove when the internal pressure is close to atmospheric pressure, but opens to admit air when the internal pressure drops significantly below the atmospheric pressure outside the container.