I have a JD 790 with a Mid-Mount Mower. The manual on the mower indicates checking the gearbox oil by removing the Allen screw on the side of the gearbox and if no oil appears, then add oil via the top of the gearbox (after removing the vent plug). Well, my gearbox has *no* vent plug on top of the gearbox. How am I supposed to add oil to the gearbox?