A shut-off valve in a fluid line closes by spring pressure and incorporates a control chamber wherein pressurized control fluid acting on a piston opens the valve in opposition to the spring pressure. A control system for the shut-off valve comprises a 3-way actuator pilot of the present invention in cooperation with a monitor pilot. The actuator pilot has a valve spool and four ports, namely, a supply port connected to a source of pressurized control fluid, a pilot port connected to the monitor pilot, a control port connected to the control cylinder of the shut-off valve, and an exhaust port for venting the control chamber of the shut-off valve. When the control system is in service and the monitor pilot senses an undue change in the pressure in the fluid line, the monitor pilot vents the actuator pilot to shift the valve spool therein to cut off the control fluid from the control port and simultaneously to place the control port in communication with the exhaust port to cause the shut-off valve to close.