This is my 2 cents worth. I have changed the Hyd filter a few times times. Somewhat frustrating as if not done just right the seal leaks. Tighten the nuts less then you think, use some of that blue sealant stuff to hold the o-ring in place and you will probably be alright, next double nut the fastening nuts to lock them on. If you use lockwashers, by the time the nut is tight enough for the lockwasher to be effective the o-ring will squeeze out and not seal properly. If the unit is cold it may appear to have sealed until you check it after a couple hours of use and see oil drooling all over. Not particularily nice when it is hot and you would rather be harvesting than fixing some pain in the butt problem and swearing up and down that they should have made the resevoir out of something besides cigarette paper thickness steel. We have a 1984 7720 which we purchased new and now has 3600 hours on it and this is about the only truly frustrating maintenance item to deal with. Otherwise the unit has been awesome. Hope this helps.