A hydraulic function disconnect means is interposed between an operator's control lever and a spring centered hydraulic control valve for a hydraulic motor actuator. The means is operable when the hydraulically actuated element approaches a limit position to prevent any operation of the valve by the lever which would move the element toward the limit position while permitting operation of the valve which results in movement of the element away from the limit position. A solenoid operated detent is used to control the ability of the disconnect means to transmit movements of the lever to the valve. The hydraulic function disconnect means forms part of the linkage in a two blocking prevention device used on boom-type material handling equipment such as a cable equipped crane in which the hydraulically actuated element comprises the movable hook and the cable is drawn in or paid out by a hydraulic motor actuator.