It is always best to replace both brushes, even if only one is worn. Worn brushes frequently are the cause behind a motor running poorly. Replacing brushes can fix a motor that's operating intermittently. New brushes can restore a motor's electric braking. Carbon brushes are one of the most commonly replaced power tool parts. This is because carbon brushes are wear items, meaning that through friction and use they will gradually wear away until they need replacing. Worn brushes are a common causes of intermittent motor function, reduced electric braking and reduced motor power. A worn-out brush will often be worn down near to the wire lead and can be cracked, pitted, or burned. When replacing carbon brushes, be sure to inspect your armature for burns, cracks, or other damage which can indicate larger motor problems.