How do you know when the carbon brushes are bad?

It does sound as if the brushes are worn. When you replace them, don't forget to clean all the contacts they press against, or the new ones will wear out very quickly.
This could also be caused by the suppression capacitor, though this is less likely. If new brushes don't fix it then replace the capacitor as well.
Also make sure the armature isn't stiff. It should rotate freely.

Colin
December 2007