to supply pressurized oil to a hydraulic accumulator 23 and hydraulic accumulator 23 is connected by a pipe 24 to an inlet of a proportional solenoid valve 25 from which extend a pipe 26 terminating in tank 19 and a pipe 27 terminating in a work chamber 28 of hydraulic actuator 15 more specifically solenoid valve 25 is designed to isolate pipe 27 and therefore work chamber 28 from pipes 24 and 26 to keep hydraulic actuator 15 in a given position is designed to connect pipe 27 and therefore work chamber 28 to pipe 24 to feed pressurized oil to work chamber 28 and move hydraulic actuator 15 in a direction 29 and is designed to connect pipe 27 and therefore work chamber 28 to conduit 26 to draw pressurized oil from work chamber 28 and move hydraulic actuator 15 in a direction 30 opposite direction 29 tank 19 hydraulic accumulator 23 and solenoid valve 25 are located at the rear axle and fitted to gearbox 10 and pipe 27 extends from the rear axle and terminates in work chamber 28 of hydraulic actuator 15 which is housed inside housing 8 of clutch 7 more specifically pipe 27 is defined by a flexible portion 31 connecting solenoid valve 25 to a connecting block 32 integral with housing 8 of clutch.