Abstract: A load-controlling piston of the double-action type, dividing an associated cylinder into two compartments, is positionable by a hydraulic servomechanism comprising a block wherein a high-pressure supply port and a low-pressure discharge port communicate with a pair of pressure cylinders occupied by respective plungers which are hydrostatically linked with the piston through conduits connecting the compartments of the piston cylinder with respective pressure chambers of variable volume formed by the two pressure cylinders. Each plunger has an axial bore accommodating a valve stem with a tapering head which is surrounded with small clearance by a valve chamber communicating at one end with the discharge port and at the opposite end via an internal passage with a gap adjacent a plunger face remote from the corresponding pressure chamber.