Brushes in a four brush starter motor are used to brush away harmful materials that can get into the motor and make it stop working. Due to how hard they work, the brushes eventually wear out and stop working, leading to a frequently stopping motor, along with a dirty interior of the motor housing. Replacing these brushes (about every 40,000 to 50,000 miles) will keep the motor strong, especially in old age.
Turn off the vehicle (whatever the motor is in) and let it rest for about two hours so everything can cool. If you don't do this, the motor will be incredibly hot and you will receive severe burns.
Look to the side of the four brush motor starter and you will see a cap that is held on with five screw. Use a Philips head to remove the screws, and take off the cap.
Use a ratchet and remove the bolts from the two large gears you find. Remove the gears to see the internal machinations of the motor.
Twist the brushes out. There will be five washers on the brush. Keep track of how they were placed on the brush, because swapping their order will not allow you to place the new brush on correctly.
Place the washers on the new brush and spin it into place. Do this for the other three brushes. Place the gears back in, and tighten the cap on the motor to finish the brush replacing.
