A hydraulic actuating device for a sliding stem control valve assembly is mounted between a control valve and an actuator housing. The hydraulic actuating device is connected to one of a control valve stem, an actuator stem, and a stem connector and the hydraulic actuating device moves either the control valve stem, the actuator stem, or the stem connector in response to varying hydraulic pressure within the hydraulic actuating device. The hydraulic actuating device may be used to override an actuator of the sliding stem valve during emergency operations, or to provide a backup actuator in the case of a primary actuator malfunction.