An oil-filled working cylinder is divided into first and second working spaces by a working piston, the first working space being connected to a high pressure chamber. A piston pump includes a pump rod received in a pump cylinder formed by the hollow piston rod, the pump rod having a bore connected to a low pressure chamber via a discharge valve, a distal end provided with a suction valve, and a down-regulating opening connecting the bore to the first working space as a function of the position of the working piston, the pump cylinder being connected to the first working space by an outlet valve. At least one of the valves is a check valve including a housing having a bore, a ball reciprocably guided in the bore by guide ribs, and a resilient retaining element capturing the ball in the bore and spring-loading the ball toward a valve seat.