The pressure wouldn't have built up overnight if the tractor was sitting. The buildup of pressure is about 10 % from 65F to 125 F, which I'm simply guessing could be the oil temperature when your driving it. 10% is about 1.5 psi, not a lot. It would certainly get by a worn seal, but I'm wondering if maybe something got wrapped around the spindle housing and damaged the seal. Now that it is leaking, I believe the only answer is to replace the seal. As I said before, I haven't done it, but it doesn't look too bad. Looking at the manual, it looks like the first step is to remove the final drive housing, which is the housing around the final axle shaft. It has a few bolts and a slot to pry it off, and doesn't have a gasket, as I recall. Then you would remove the spindle housing from the main axle housing. Disassembly requires removing some snap rings and pressing the gears and bearings off. If you keep track of the shims, it should go back together (press) without worrying about measurements. The manual actually shows two seals on the drawing, and maybe the outer one is like a dust cover to prevent stuff from getting into the real oil seal. If you find seepage behind the axle/wheel flange, you should replace the outer seals while you have it apart. I read the thread about the breather hose, and agree that the oil wouldn't go out unless it was overfilled, or at a large angle, allowing oil to get into the hose. The suggestion to use two breather hoses is a good one, but you'd have to drill a hole in the left side axle housing. Not a big deal, but not sure how you can avoid getting chips in the housing. Use grease, or a vacuum, and also a magnet afterwards, but drain the oil first.
