Those brushes are just about played out. As stated it's best to replace them while you have it apart.
My 6v motor was froze up when I started working on getting some heat. I broke the heater down and tore into the motor. I was lucky in that my brushes were hardly wore, I guess they didn't run the heater much back in the day,
The shaft at the commutator end was a little rusted and burred where it inserted into the bushing. I shined both ends up w/some emery cloth and put a small dab of grease in both bushings.
I also shined up the commutator a little w/emery cloth and cleaned everything up.
That freed it up and got it working again.
Good luck
Dave
