Motor shops use a seating (chalk)stone, I've always sanded brushes and let them run on hand overnight and/or for a day or two like uppo72 suggested. I've heard of older Dover and Otis adjusters making helpers seat generator brushes with sandpaper and ONLY pull in the rotation of the commutator direction. They sell commutator sandpaper at Martindale I think, for seating brushes. It's not supposed to get embedded in the brush.
