Hey guys,
Even if you only have one system, if your reservoir was designed like mine, it could be leaking into the transmission, or worse yet, the engine. The fluid has to be going somewhere. Using cardboard to check for a pressure leak will reveal a leak. If you cannot find a pressure leak, and, if it's not going on the ground, which it sounds like it's not, then the fluid is somewhere in the tractor.
Where is the sump/reservoir located? If it's mounted to the top of the tranny/rear-axle it could leaking into those areas. I was trying to suggest that be looked at. There could be a crack in the housing or something else leaking into an adjoining area.
Good luck with this. I know how aggravating it can be.
Thanks,
Eddie