have you made the MISTAKE of releasing the brush OUT before trying to fit it most brushes are sold to you in there retracted position hence easier to fit, there are 2 types(1) bolt direct to motor (2) slide up the slot type held by a (plastic bendy clip that must enter the motor casing to hold the brush in place)
Fit either type FULLY (before releasing the tag) on the side of the brush holders this also prevent the brush missing the com and snapping the brush
wirecutterman
February 2008