Not sure what part exactly is leaking on your Kubota (do not know squat about them little fellows,myself). Had a leak at the bottom of the steering column on my CIH 1494,at the steering hand pump.Had a hell of a time finding it,took two people to finally figure it out,one to turn the wheel,one to watch the pump.The pump would only leak when warm, under pressure,and when turned about 1/2 way to one side only.If you did not turn it enough or too much it would not leak.Your air test would not have found this problem either.I've often found that it is next to impossible to simulate actual use situations with the part not in place artificially,myself.The hand pump was repaired with new o-rings,and a clean-up of mating surfaces.I would suggest against using heavier oil in the steering system,it might just get quite costly!!!