I had the same issue w/ a trans a while back. Mine got in via the fill tube somehow. I still had all forward gears it just slipped like heck. I drained it at the track and refilled it. It still slipped some. The only way to do it right is to pull the trans and drain the converter too. OR unbolt the trans line from the cooler, start the car, put it in N and run the car until you see nice fluid pumping out. I pulled my trans, drained the covert, dropped the pan, and used a leaf blower to get all the water out...might have been overkill.