You may need to pump the Topsider more to maintain vacuum.
I have found that you initially have to pump about 40 strokes. After the Topsider has sucked oil for 5 minutes or so, pump the handle a few times and see if the resistance is about the same as when you started. If you feel less resistance then you need to pump 10 or 20 more times to keep vacuum.
You may have to do this 2 or 3 times during the 15 or 20 minutes it takes to drain out the oil.
Bill Wood - Webmaster