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How do you know when the carbon brushes are bad?

The brushes on my shop vac motor are making a pretty big green spark and the motor seems to be slowing down.
Brad, December 2007
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