Abstract: A differential piston is reciprocable in a cylinder and subdivides the latter into a first and a second chamber. The piston has a piston rod which extends through the second chamber outside the cylinder and is adapted to open and close the mold of a molding machine. An admitting conduit admits pressure fluid into the first chamber, and a connecting conduit connects the two chambers and has a valve interposed in it so that the two chambers communicate with one another only while the first chamber is pressurized, not while the second chamber is pressurized in the absence of pressure in the first chamber. A sealing arrangement between the piston and an inner circumferential surface of the cylinder is effective for preventing the flow of pressure fluid from the second to the first chamber when the latter is at a lower pressure than the former.