Pauric,
Welcome
You need to make sure you have the proper filter, i.e. one that does NOT have a bypass, like automotive oil filters do. Some of the sponsors above carry them and so should your local cub cadet dealer. There are substitutes Napa, Wix, AC Delco etc., but make 100% sure there is no bypass.
The leak could be coming from above the filter. The release valves and pump to rear end interface gasket are a common places. Clean the hydro/rear end unit off with a degreaser and wipe down. The hydro must be very clean to find the leak location. Two of my hydro’s leak, just not a lot, so I put it off because fixing hydro leaks on narrow frames (127’s and older units) is a pain.
The fluid level is checked on the rear cover plate on the back of the rear end. There should be a pipe plug near the middle of the plate. The fluid should be up to the bottom of this hole (tractor not running).
The hydro unit / brake setup as an adjustment for this creeping problem. You should try to get a service manual which will cover this adjustment. Sorry I can explain too much to type on this one.
Someone might post the fix from the service manual they have scanned.
Jim