Finally got around to working on the truck. I wanted to change the fluid and filter and since I had the pan off I dropped the valve body to check things out. I took a fender washer with a vacuum hose that fit snug in the hole. I attached a blow gun on the other side of the hose. I held it tight on the OD port on the rear of the case and the OD piston did not move but I heard a gush of air in the housing. I assume the OD piston seal is bad.