komatsu d65 steering problem

Hi I have a komatsu d65e that has foamy oil and requires a lot of bleeding to get the steering working. I chnaged the oil and checked the suction lines but I cannot find any leaks. Is it possible that the pump itself is taking in air? If I can get it bled it will steer for a while and then I need to bleed it again, so it is getting a lot of air. Is the foaming of the oil due to water contamination ? If so, from where ?
thanks in advance.
john

komatsu d65 steering problem