Mine is a later model ('07), and does have the filter guard. As folks have suggested, you should get the retro-fit, when you get the tractor back on line.
I've seen some 2300 tractors where the plastic fill cap fits well and unscrews by hand, but mine sure isn't one of those!
I have to use a pair of Channel Locks on it every time; either the threads were poorly formed or have been damaged, I suspect, in either the tube or the cap.
I don't worry too much about it, as long as it stays put-- but go ahead and use a tool to get it off. When wandering around dealer lots, I got in the habit of just trying those caps; seems like only one in four or five was actually removable by hand. I know it's a bear to get at with the BH on!
Wish I could help more with your hydraulics, but I'm still learning, too, and have never had that kind of a problem with mine -- unless it was an obvious mis-match of QCs.:laughing: Thanks for keeping us posted.