Hydraulic system for pressing workpieces (11) comprises a pressure transfer device (10) having a housing (11), a first piston (12) which divides the housing (11) into two pressure chambers (13,14), both chambers being connected to a pressure source (15). The other side of the piston has a rod (25) whose work surface (26) is smaller than that of the main piston (17). This surface makes a work area (27) for the hydraulic fluid and as it moves forward it builds up pressure. The novelty is that a valve (35) is present which connects the two chambers (13,14) during pressure build-up until a certain pressure is achieved or the piston (12) has moved a certain distance. During this time fluid flows from (14) to (13).
