3WITH the rotor assembly taken out, take light-grade sandpaper to the commutator (copper surface the brushes run against). Good contact is vital here, so the surface should be as clean and smooth as possible. While there, use a small pick or screwdriver to clean out the grooves in the commutator.
Before re-installing the rotor assembly, clean it and the housing with contact cleaner. When you’re sliding the rotor assembly in, the brushes on the housing will be in the way. With a small, flat-blade screwdriver, hold each brush back just enough to let the commutator slide past.