JOE Junion, a Reliability Specialist for L&S Electric, has written about the importance of correctly seating DC motor and slip ring brushes.
According to Junion, if the commutator and the brush are not making good contact with each other, the surfaces of both can be damaged by sparking.
He says commutator maintenance costs can be very expensive and may involve tuning and undercutting.
Brushes are designed to conduct current to the armature circuit as well as short the commutator bars. If the brush is not correctly seated, it will not make full contact with the full face of the commutator and will not short its bars.
Junion says the best way to seat brushes is to use a piece of sandpaper which is 30cm long and is slightly wider than the item’s width. The paper is slid back and forth under the brush until it is in place.
The sandpaper must be kept flat against the commutator so the brush face edges are not rounded. 
He says technicians cannot use standard garnet emery sandpapers. They must use paper with a grit size of at least 80.