1997 eldorado. Changing the anti freeze by draining the radiator, seems to get only a gallon or so out. I did this last year and added the crushed tabs to the lower radiator hose. If I drain out the same amount this year, to try to "purge" the old anti freeze out, do I still have to add the stop leak pills? I mean, when is too much too much with the sealant pills?