Common issue with Kubota tractors (all models) is the filter gasket sticking to the filter boss on the gearcase casting and then you put on a new filter and the old gasket and the new filter gasket against the old one compromises the seal and if the pump sucks any air (from the double gasket), it looses it's prime.
On my M9's it hard to see of the old filter has left it's gasket stuck to the gearcase as the filters are up, underneath the operators station so you need to physically look at the removed filter to see if the gasket came off, or 'feel' the gearcase boss to make sure the old gasket wasn't left behind. It even states that in the owners manual, least mine does.
Because both my M's hold a lot of fluid (almost 15 gallons each), draining them can be a real mess, no ordinary drain pan works, so what I did was, I went to the scrap yard and bought an old aluminum truck fuel tank (you can use steel or aluminum) and I cut one end off with about a 12 " high wall and I drain my fluid into that. Handles the mess quite nicely. Welded handles on each side too (optional) and I siphon the used fluid back into the 5 gallon pails the new fluid came out of and 'donate' them to my dealer, he has a waste oil burner in his shop.