Patent ID: 7108059

Claim:
A downhole pump assembly, comprising: a housing having a longitudinal axis extending from an upper end of the housing to a lower end of the housing and a chamber extending through the housing from the upper end to the lower end, the housing having an upper internal shoulder and a lower internal shoulder, the lower internal shoulder having an upper surface substantially normal to the longitudinal axis of the housing, the upper and lower internal shoulders arranged in an opposing relationship; a standing valve located in the housing to permit one way flow of fluid into the chamber of the housing; a plunger disposed in the chamber of the housing above the standing valve and below the upper and lower internal shoulders thereof and adapted for reciprocating movement through at least a portion of the chamber of the housing; a traveling valve located in the plunger to permit one way flow of fluid into the plunger; a pull rod having one end connected to the plunger and an opposite end connectable to a sucker rod string to affect reciprocating movement of the plunger; and a sliding valve positioned in the chamber of the housing between the upper internal shoulder and the lower internal shoulder, the sliding valve moved to an upward position into engagement with the upper internal shoulder in response to upward movement of the plunger, in the upward position the sliding valve permitting fluid to flow around the sliding valve and upwardly through the chamber of the housing, the sliding valve moved to a downward position into engagement with the upper surface of the lower internal shoulder of the housing in response to downward movement of the plunger, in the downward position the sliding valve restricting the downward flow of fluid through the chamber of the housing sufficiently so that the pressure above the traveling valve is less than the pressure below the traveling valve during the downward movement of the plunger thereby causing the traveling valve to open, wherein the pull rod is provided with a portion of reduced diameter extending through the sliding valve near the end of the downward movement of the plunger thereby increasing the flow area between the pull rod and the sliding valve to permit the pressure above the sliding valve and the pressure below the sliding valve to substantially equalize prior to commencement of upward movement of the plunger so that the sliding valve is caused to move in the upward direction upon upward movement of the plunger.