I have seen some comments that indicate that the odd sparking is not a problem.
When the brushes are worn, I believe, you will get erratic behavior from the router.
So far, through limited use, I have not had to replace the brushes in any of my routers.
When I installed the MuscleChuck on my Triton TRA001, I removed the brushes while removing the old collet.
I had this router for some years and the brushes were hardly worn.
Others may have varied results...
I don't mind if other members disagree with my comments.
I don't profess to know everything, and I may learn something new.
"Never attribute to malice that which is adequately explained by stupidity."