Abstract: A chamber of an actuating mechanism is hydraulically connected to the under-the-piston cavity of the cylinder adjacent to the housing of a pulsator. Built into the piston is a spring-loaded valve serving as a device for compensating for the leakage of liquid from the chamber and for protecting the latter against overload pressure. During the operating stroke of the piston, the valve is brought in contact with it by an eccentric, and in case the pressure is increased or it is necessary to compensate for the leakage of liquid, a clearance is formed between the valve and the piston for the liquid to pass the direction of the liquid flow being dependent on the pressure differential on the valve.