Abstract: A pneumatically operated hydraulic pump includes a cylinder housing accommodating a main piston to subdivide the housing interior in two working chambers. The main piston is connected to a hydraulic piston for joint reciprocation so that the hydraulic piston can carry out a suction stroke when the main piston moves to one end position and a pressure stroke when the main piston moves to the other end position. Arranged laterally to the cylinder housing is a control mechanism for regulating a flow of working fluid alternately to the working chambers. The control mechanism includes a plastic valve casing for accommodating a control piston having opposite end faces of different size to define different effective areas and thereby realize reciprocation of the control piston by the working fluid. A slide shoe is guided in the valve casing for conjoint movement with the control piston.