Abstract: The device (D) for controlling the reciprocating displacements of a differential piston (1) includes at least one valve associated with each of the parts of different cross-sections of the piston (1), and a rocker mechanism (B) urged by a spring in a direction transverse to the direction of displacement of the piston (1). The rocker mechanism (B) is designed to control the valves in closure and/or opening at each end of stroke in order to reverse the movement of the piston (1); a push rod (55) is mounted so that it slides relative to the piston (1), whilst being able to be driven by this piston, the push rod being capable of coming to bear, at each end of stroke, against a limit stop (s, 14a) connected to the enclosure, in order to give rise to the rocking of the mechanism (B).