Take a flashlight and look in the brush holder opening. Slowly rotate the motor rotor and make sure none of the insulators (between each commutator bars) are sticking up. The rotor commutator should be smooth. Carbon brushes normally don't break like yours unless being struck by a bad rotor bar or insulator. The commutator should be a light to medium brown color. Any blackened or burned bars are from "pin arcing" that usually indicates a shorted rotor winding.