An air driven reciprocating piston hydraulic pump having a main changeover valve with a main valve body having an annular valve body portion which includes first and second annular valve faces that approach closely or contact first and second annular valve seats on valve cases which are formed on the upper and the lower ends of the annular valve body portion. An air intake chamber is defined by a piston reception hole and a drive piston portion, and an auxiliary changeover valve changes over the main changeover valve between its air supply position and its air discharge position by supplying compressed air to the air intake chamber or discharging air therefrom via a first and second valve members and a small diameter portion of a valve rod. A center side portion of the main valve body is made as a separate auxiliary valve body for facilitating up and down motion thereof.