What is the function of slack adjusters in an air brake system?

Slack adjusters connect the brake chamber push rod to the brake camshaft. Their job is to take up the slack that develops as brake linings and drums wear, keeping the brake stroke (the distance the push rod must travel to apply the brakes) within specification. If brake stroke becomes too long, the brakes may not apply with full force in an emergency. Modern trucks use automatic slack adjusters that adjust themselves under normal operation, but they must still be checked at every pre-trip inspection because they can fail. Manual slack adjusters require periodic manual adjustment. CDL drivers should not adjust manual slack adjusters in many states — it is mechanic's work — but they must know how to check stroke and identify out-of-adjustment brakes during inspection. An out-of-adjustment slack adjuster is one of the most common citable defects on a roadside inspection.