This is the steering valve that is on my M5. Your M670 should be basically the same. There were some changes on the early M5 valves and the later M5 ones, mainly O ring placement in the valve its self. You could possibly try a thicker O ring, unless somehow you are getting oil pressure leak when the valve moves. Does it leak without moving the steering wheel? These always seem to be a trouble spot on these tractors. Ive heard stories of some of them not work properly when new.