Abstract: A valve assembly senses the cumulative volume of hydraulic fluid that has passed to an actuator and senses and subtracts from that cumulative volume the volume of return fluid that has passed from the actuator. When the difference between the volumes exceeds a preset value, the system shuts off the flow to the actuator to prevent further loss of fluid from a leak. The assembly may employ separate sensors in separate pressure and return lines or a common line with a common sensor which reverses signal polarity when flow reverses. Target flowmeters with signal adding and subtracting circuits may provide the differential volume information.