a valve body including a chamber, located on an axis, having axially upper and lower ends and upper and lower orifices located at the upper and lower ends through which air can exit the chamber;

an upper plug configured, in a closed position, to close the upper orifice;

a lower plug configured, in a closed position, to close the lower orifice;

a spring compressed between the plugs so as to elastically urge the plugs axially apart and into their respective closed positions; and

a plug displacement device configured to displace the upper plug from its closed position against bias of the spring into an open position, and to displace the lower plug from its closed position against the bias of the spring into an open position;

wherein the plug displacement device comprises a valve stem extending axially through both orifices and having upper and lower abutment surfaces, the upper abutment surface being located above the upper plug and configured to push the upper plug downward into its open position, and the lower abutment surface being located below the lower plug and configured to push the lower plug upward into its open position; and

both abutment surfaces simultaneously abut their respective plugs when the plugs are in their closed positions, such that the biasing of both plugs into their closed positions by the spring biases the stem into a neutral position.