Having a bad motor brush will make the motor not work all together. This can be very frustrating when troubleshooting the problem. Having the ability to find the problem and fix it in this case is very important and will cut down on the time spent troubleshooting your next electric motor problem. There are several key things that you should look for in order to tell whether or not the problem you are experiencing is due to the brushes not working.
Use your voltage tester to see if electricity can go through the electric motor. If it can, then the motor is operable and perhaps your power source is the cause of the problem; if not, then it is the motor that is at fault.
Look inside the electric motor through the little holes on the end of the motor at the metal wires (brushes) that are wrapped around the inside. If it looks dirty inside, or if the wires are broken, then you may have a bad brush.
Remove the brushes if they are broken. Clean the housing and replace with new brushes.
Test the motor again with your voltage tester after replacing the brushes and cleaning the housing to see if your fix has worked.
When working with smaller electric motors, most of the time it is more cost effective to buy a new motor.