JD tractor question - seller and others

I've got to pick the brains on some Deere guys. I
had some odd hydraulic things going on with the
7800. A while back I had transmission issues and
ended up putting a wheel speed sensor in it and it
was fine through planting. During baling there were
some odd hydraulic pressures. When it is cold it is
fine but once it had been run for a good while it
would kick the transmission out if you used the
clutch. The clutch would also nearly rip your head
off if you used it and was inconsistent. It also blew
apart a door cylinder on the baler and the last fields
planted showed signs that it wasn't applying down
pressure like it should. I finally realized the fluid was
getting hot and the air pump was not returning fluid
to the rear.

I cut the air line on the left side of the transmission
and the pump is working (sucking air). I am able to
both blow and suck through the line going back to
the rearend so it isn't blocked (this is the model
where the pump isn't vented - it is a continuous loop
to the rear diff). The machine still never returns the
fluid to the rearend. It is right at the full line before
starting and is barely visible in the sight glass no
matter how long it runs. When you shut it off the
level slowly rises back up.

What are my next steps?

I don't know about your tractor but my buddy had a 185ac that would add fluid to the tranny from the hydro reservoir. We called a mechanic and he said they had one that did the same thing and the quick fix was to put a line from one drain plug to the other so the fluid would level out. But under constant use it would still get out of wack a little. The correct fix was splitting the tractor and putting in new seals. We did the cheap fix and that was about three years ago and it's still that way.