摘要：

A dispensing pump having upper and lower pistons defining opposite ends of a two diameter pump chamber, the volume of which is varied by movement together of the pistons, while a small amount of relative movement of the pistons, resulting from pressure changes within the pump chamber, effects opening and closing of the discharge valve. For facilitating the priming of such a pump, the lower piston and cylinder are arranged to define a normally closed priming valve operable at a predetermined point in the lower piston stroke to permit reverse flow of entrapped air from the pump chamber into a liquid supply container with which the pump is associated.

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