Sometimes just turning the brushes around will fix the issue if the brushes are the problem.  It also won't hurt to just lightly scuff them with some fine sandpaper in case there are some contaminants that have cooked onto them from the friction.  Also inspect the commutator (where the brushes rub on the motor shaft).  If it is damaged or particularly dirty, that could also be a sign that the brushes are related to the problem.  

--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.