1. IN A HYDRAULIC SERVO MOTOR MECHANISM COMPRISING A CYLINDER, A PISTON RECIROCABLE IN THE CYLINDER, THE CYLINDER HAVING A FLUID INLET AND A FLUID OUTLET, ONE ON EACH SIDE OF THE PISTON, PASSAGE MEANS TO DIRECT HYDRAULIC FLUID FROM SAID INLET THROUGH SAID PISTON, A CONTROL VALVE MEMBER AND A CO-OPERATING VALVE SEAT TO CONTROL THE FLOW OF HYDRAULIC FLUID THROUGH SAID PASSAGE MEANS, AND AN INPUT MEMBER CONNECTED TO SAID VALVE MEMBER FOR MOVING VALVE MEMBER TOWARDS SAID SEAT, SAID PASSAGE MEANS BEING ARRANGED TO TRANSMIT THE FLUID PRESSURE IN SAID INLET TO ONE SIDE OF SAID VALVE MEMBER IN SENSE TO MOVE SAID VALVE MEMBER AWAY FROM SAID SEAT IN OPPOSITION TO CLOSING MOVEMENT OF SAID VALVE MEMBER, THE IMPROVEMENT WHICH COMPRISES A COLLAR MOUNTED UPON SAID INPUT MEMBER AND AN ABUTMENT ON SAID INPUT MEMBER ENGAGEABLE WITH SAID COLLAR, SAID VALVE SEAT COMPRISING AN ANNULAR VALVE SEAT FORMED ON SAID PISTON IN AXIAL ALIGNMENT WITH SAID COLLAR, SAID PASSAGE MEANS INCLUDING A PASSAGE EXTENDING LONGITUDINALLY THROUGH SAID INPUT MEMBER AND HAVING ONE END IN COMMUNICATION WITH SAID INLET AND ITS OTHER END IN COMMUNICATION WITH A SPACE BETWEEN SAID PISTON AND SAID VALVE MEMBER AND WITHIN SAID VALVE SEAT WHEREBY COMMUNICATION IN BETWEEN SAID INLET AND OUTLET IS CUT OFF WHEN SAID COLLAR IS MOVED AXIALLY INTO ENGAGEMENT WITH SAID VALVE SEAT.