After working on my G1 and X1 I was trying to ascertain when you replace the brushes? Do you have to remove them to see?
I have noticed a lot of carbon build up on the X1 around the belt area and the cover. I did read that they have a tendency to extrude a great deal of carbon.
I couldn't see any obvious way of seeing how worn they are. Also, if the process is taking them out and measuring them, at what point should you consider replacing them?