A hydraulic pump for adding a predetermined volume of additive fluid to a primary fluid, the pump comprising: a main body having a first inlet means for receiving a primary fluid, a second inlet means for receiving an additive fluid, and an outlet means for discharging a fluid, wherein the fluid comprises a fluid mixture of the primary fluid and the additive fluid or the primary fluid; a piston sealingly mounted in the main body for reciprocating movement in response to flow of the primary fluid through the main body, the piston dividing the main body into a first and a second chamber (238, 240); and an extractor means attached to the piston and sealingly mounted in a third chamber formed in the main body, the extractor means being slidably mounted in the third chamber such that the extractor means reciprocates in response to movement of the piston for pumping additive fluid from a source of additive fluid through the second inlet means to the second chamber; characterized by a first valve means for selectively transmitting the primary fluid from the first inlet means into the first and second chambers (238, 240); a second valve means for selectively transmitting the fluid from the first and second chambers (238, 240) to the outlet means; and means for operably interconnecting the piston and the first and second valve means (204, 206) comprising a linkage means and a spring means attached thereto and responsive to the reciprocating movement of the piston for alternating the first and second valve means (204, 206) between a first state, wherein the first valve means transmits primary fluid from the first inlet means to the first chamber and the second valve means transmits fluid from the second chamber to the outlet means, and a second state, wherein said first valve means transmits primary fluid from the first inlet means to the second chamber and the second valve means transmits fluid from the first chamber to said outlet means. a main body having a first inlet means for receiving a primary fluid, a second inlet means for receiving an additive fluid, and an outlet means for discharging a fluid, wherein the fluid comprises a fluid mixture of the primary fluid and the additive fluid or the primary fluid; a piston sealingly mounted in the main body for reciprocating movement in response to flow of the primary fluid through the main body, the piston dividing the main body into a first and a second chamber (238, 240); and an extractor means attached to the piston and sealingly mounted in a third chamber formed in the main body, the extractor means being slidably mounted in the third chamber such that the extractor means reciprocates in response to movement of the piston for pumping additive fluid from a source of additive fluid through the second inlet means to the second chamber; characterized by a first valve means for selectively transmitting the primary fluid from the first inlet means into the first and second chambers (238, 240); a second valve means for selectively transmitting the fluid from the first and second chambers (238, 240) to the outlet means; and means for operably interconnecting the piston and the first and second valve means (204, 206) comprising a linkage means and a spring means attached thereto and responsive to the reciprocating movement of the piston for alternating the first and second valve means (204, 206) between a first state, wherein the first valve means transmits primary fluid from the first inlet means to the first chamber and the second valve means transmits fluid from the second chamber to the outlet means, and a second state, wherein said first valve means transmits primary fluid from the first inlet means to the second chamber and the second valve means transmits fluid from the first chamber to said outlet means.