+1 to what henchard said (if your tractor has no return or there's a problem using it). I used to do this with my old Kubota that didn't have a return when I needed it to run the bale wrapper. One point - my father in law (retired farmer) told me off for this. Apparently hydraulic fluid takes on water if left open to the air, causing problems (plus an open filler will allow debris in). So I made an adapter that temporarily replaced the filler cap and allowed the return coupling to plug into it. It was quite a few years ago now & was a bit heath robinson, but did the job.