The diagram below shows the proper clearance for brake shoes on the 49-54 brake system. The primary shoe is supposed to contact the drum at both the top and bottom of the lining material. The secondary shoe should have .015 in. clearance top and bottom. The primary shoes can be adjusted as shown due to the normal rotation of the brake drums, it tends to push the primary shoe away. The secondary shoe must have clearance because drum rotation tends to pull this shoe into contact with the drum. This is what is known as “self-energizing” action and the mounting of the brake shoes is intended to enhance this effect.