What is a slack adjuster in an air brake system, and why does it need to be properly adjusted?

The SLACK ADJUSTER connects the brake chamber pushrod to the brake S-CAM (which spreads the brake shoes against the drum). It 'adjusts the slack' to keep the proper clearance between the brake shoes and drum as the linings wear. CDL air brakes. WHY ADJUSTMENT MATTERS: if the slack adjuster is OUT OF ADJUSTMENT (too much travel/free play), the pushrod travels too far, the brakes don't apply fully, and braking effectiveness is REDUCED — a serious safety hazard (a common cause of brake-related violations and crashes). AUTOMATIC slack adjusters are now standard but still need to be checked (and shouldn't have to be manually adjusted often — if they do, something's wrong). Knowing the slack adjuster's role and the danger of misadjustment (reduced braking) is commonly tested.