Automatic slack adjusters keep a constant clearance between your brake linings and brake drum, approximately 25 thousands of an inch. The push rod travel stays at one and a half inches when the slack adjusters are working properly.They can be adjusted manually (only during installation) and automatically by the operator: by performing a full brake application[the air pressure must be over 90 p.si.] with brakes hot. This will assure proper adjustment, the slack adjuster automatically ratchets to take up any slack in the brake push rod travel. These slack adjusters [manufactured by HALDEX]are very reliable but do wear out causing no auto adjustment which will be obvious during a brake check. During a service we check the push rod travel on a regular basis, every 3000 km. [1800 miles] The push rod travel must not exceed 2 inches. For a fleet operation auto slacks are the way to go since continually adjusting air brakes can take a lot of man hours. Auto slacks are not supposed to be adjusted at all, according to a recent bulletin. If they go out of adjustment there is something wrong with the slack itself or the foundation brakes.