Yes that is correct. If your linkage worked then I would think it is an airlock. I would assume all the tractors would be the same
Make sure you are higher with the transmission. It needs to be pretty steep, but as soon as your high enough it should work. I tried putting oil back through the line with a curved hose. That was hopeless because I realised the oil was just flowing back in to the transmission and only a dribble would be left in the filter. New ones must self prime through the gear pump, I don't know. Or they fill it with a vacuum pump. I dont think there is anything on it in the manual. The outlet from the filter is on top of the pump, crack the three bolts if you need to for it to bleed. Good luck and let me know how it goes.
