strip the brushes out of the motor check there length , they may be just sticking in the brush holder, if on the other hand very little carbon remains 1/4" or 3/8" invest in a new set of brushes these will be about 1"to 1 1/4" in the holder
DO NOT RELEASE THE BRUSHES FREE TILL FITTED TO THE MOTOR this is cheaper than a new motor.
wirecutterman, August 2008