you replace the brushes if they become discolored or if they become half the length of a new brush. When the brush becomes discolored it actually is much harder and looses its lubrication that is built into the brush and it is much harder on the comm wearing grovves in faster. Discoloration can be caused by motor overheating... make sure you do the 5 second finger rule. Touch the can for 5 seconds if it hurts to touch it or you can't hold your finger there it's too warm go down on pinion size.