Abstract: The annular transmission block (13) is pressed against the transmission face (8) of the cylinder (5) when the oil pressure arrives via the duct (17). At that moment, one of the two grooves (19 or 10) can supply one of the two ducts (9 or 10) in such a way that the oil pressure is transmitted across the fluid-retaining devices (11and 12) to one of the two ends of the chamber (4). The pressure-maintaining device (25) is then charged by stressing its spring (27) and the piston (6) moves into the chamber (4) of the cylinder, carrying along the clamp chuck (7).