Abstract: A reversible, positive displacement, expansible chamber hydraulic pump with only three moving parts. Intended for manual operation where the creation of high fluid pressures is desired, the pump consists of a rotating input shaft with a single eccentric cam. The cam operates both a piston and a slide valve which are located at a relative angle of ninety degrees from one another about the axis of the cam. The slide valve controls fluid flow to and from the pump cylinder such that fluid flow reverses upon reversal of rotation of the input shaft and such that fluid flow is blocked when the piston is at top or bottom dead center.