The breather tube should have some type of filter on the top of it so you might start there making sure it is not plugged in some way. The theory is that as the oil in the rear end gets hot it expands and would build pressure in the transaxle and if there was no vent it would push oil out around the seals or the easiest place it can find. Also as it cools the oil becomes smaller in size and it needs air from the outside to prevent a vacuum and that is where the filter on top of the vent hose comes into play, lots of dirt flying around on a lawn mower when mowing .One grain of sand don't hurt a hydro ,it destroys it. Also if your fan has lost any blades or you don't run your Engine speeds fast enough the oil in the rear end will get very hot and as it cools it will condensate on the inside in the oil, like a cold bottle of soda pop on the table in the summer as the moisture always goes to the warmest side. With time it gets enough water in the oil to start foaming and the oil gets very thin and will boil a lot quicker then 20w50 motor oil.

If it is a breather tube and pressure is building up , it is possible that there is oil in the tube and combined with dust it has gone solid and is blocking the breathe
check for any low areas of the hose that might retain oil