This question is in the Smart Media category but the Greenmaster GMD 118 appears to be a treadmill.
Carbon brushes are usually held in place by a securing device - you do not have to dismantle the motor to get access to them. On this particular motor the brushes are held in place by a plastic screw cap - with a screwdriver slot.
Google images will show you pictures of the motor and whee the brushes are located.
Amazon will sell you replacement brushes.
Brush failure is usually suggested by - smell - noise and sparks. If your motor fails to run the cause may not be the brushes.