A pump dispenser connected to a container of liquid has a first piston reciprocatable in a first cylinder for pumping a quantity of liquid at each depression of the piston. A valve member received within a channel in the first piston is spring biassed into a position in which a dispensing outlet is normally...http://www.google.es/patents/US5458289?utm_source=gb-gplus-sharePatente US5458289 - Liquid dispensing apparatus with reduced clogging
