an air - actuated disc brake develops the required force magnification to operate disc brakes from an air pressure chamber using push - pull rods driving a pair of cam levers applying normal force to one shoe plate of the disc brake . clamp action is carried through to an opposing brake shoe plate by a caliper . the entire air applied disc brake is mounted as a unit to the axle flange . an automatic adjuster compensates for brake lining wear . the cam levers are pivotally mounted to a movable cam carrier located between the caliper and one brake shoe plate .