A worn axle bearing that allows the axle to bounce up and down on the seal, or a worn surface on the axle shaft where the bearing rides is the most common cause of a leaky seal. Try to move the axle shaft up and down with the brake drum removed. Anything other than a very tiny up/down movement is too much. If you can see the axle move up and down, replace the bearing and/or the axle as well as the seal. There's no way to get all the oil out of the brake linings, also.
Jerry
