whenever the pressure of the hydraulic fluid as applied through orifice 22 is sufficient to overcome the closing force of spring 24 the orifice 22 and its function will be more fully described hereinbelow to initiate fast closing of the steam valve 17 a disc dump valve 25 comprising valve disc 26 pilot chamber 28 spring 30 and drain passage 32 is provided to very quickly release the hydraulic pressure in chamber 14 and thus cause closure of the steam valve 17 pressure in chamber 14 is released either by activating solenoid valve 34 to connect pilot chamber 28 through the solenoid valve 34 to the drain or by leaving solenoid valve 34 deactivated as shown and releasing the pressure applied by the emergency trip hydraulic fluid supply in the latter case fluid from pilot chamber 28 flows through solenoid valve 34 and check valve 36 to the emergency trip supply operation of the disc dump valve 25 is more fully described in the above cited patent u s pat no 3 495 501 the disclosure of which is incorporated herein by reference thereto to isolate hydraulic components of the actuator system from the hydraulic fluid supplies to permit maintenance or repair to those components an isolation subsystem is provided comprising manual isolation valve 38 orifice 22 and check valve 36.