Here's a reply from Jim from the archive site:
"99% of the problems with a 3100 error are related to the brake parts/motor housing being DIRTY. usually a good test if the motor housing is dirty: press on the moveable arm that comes in contact with the housing, if it sticks , even one a second, that would cause the error.
so even if the parts look GOOD, if there dirty that's your problem. disassemble and clean the parts (especially where the pivot action takes place).
you will see the difference after you clean them, that includes the red part as well, clean the inside of the red part as well as the metal part it pivots on.
I'll bet your problem will go away.
Jim"
Dave