Hi, Just had the Kohler motor rebuilt on my Case 220 tractor, reinstalled it and changed the oil in the rear end with about a pint of Valvoline 20W50 oil and also changed the hydraulic oil, replacing it with Rotella 15W40 oil, filling the reservoir to within 6 inches of the top of the filler neck per the manual. When I operate the tractor now, the engine loads up and bogs down when travelling in the forward or reverse direction. This happens after running the tractor for about 5 minutes or so. The engine does not seem to bog down when I just run the engine but do not engage the hydraulic drive, only when the hydraulic drive is engaged. I am wondering if anyone here has any idea what might be causing this? I followed the directions in the manual for changing the hydraulic and rear end oil but I am wondering if maybe I have gotten air in the hydraulic system while pumping out the old hydraulic oil or something like that. If anyone has any experience with this situation, would appreciate your thoughts on the subject. Thanks in advance.