A piston pump for dispensing foam by simultaneously dispensing air and liquid through a foam generator and in which air to be dispensed is drawn into a chamber from which it is to be dispensed, at least in part, through an air inlet passageway which is different than a flow path out which foam is dispensed. A lost link valve arrangement provides for relative motion to open the air inlet passageway when air is drawn into the chamber and to close the air inlet passageway when air is forced out of the chamber to a dispensing outlet. Preferably, the lost link valve arrangement is provided by a piston carrying an actuator member coaxially slidable thereon to selectively open and close the air inlet passageway.
