(under subclass 437) Apparatus in which the means for controlling pump fluid to or from the pump chamber includes valve means of the type which normally opens or closes responsive to a pressure differential between the pump chamber and an inlet or discharge conduit for cyclically
controlling pump fluid for the pumping operation and including means operable as a result of the pumping member being moved to a given position in the pump chamber for moving said valve to open position irrespective of said pressure differential.
(1) Note. A pressure responsive distributor which can be selectively opened only when the pumping member is at a given position in the pump chamber is included under this definition, e.g., a pressure responsive foot valve which is capable of being opened by relative rotation of a reciprocating pumping member when same is at the end of its reciprocatory stroke.
