Carbon motor brushes are one of the most commonly replaced power tool components. Because carbon brushes are wear items, meaning that through friction and use they will gradually wear away, they will eventually require replacement. Worn brushes are common causes of intermittent motor function, reduced electric braking, and reduced motor power. A worn out brush will often be worn down near the wire lead and may be cracked, pitted, or burned.
When replacing carbon brushes, be sure to also inspect your armature for burns, cracks, or other damage which indicates a larger motor problem.