Final answer:
Automatic star wheel adjusters in drum brakes adjust when brakes are applied in reverse, and during a firm brake pedal application at a standstill. They compensate for wear and maintain proper shoe-to-drum distance for effective braking.
Step-by-step explanation:
Automatic star wheel adjusters in a drum brake unit perform the adjusting action during two primary instances: when the brakes are applied while the vehicle is moving in reverse, and when the brake pedal is pressed while the vehicle is stationary, but with greater force being applied. This is because the adjuster is designed to compensate for brake shoe wear and maintain the correct distance between the brake shoes and the drum. The adjustment mechanism usually includes a lever that rotates the adjuster when the reverse maneuver or firm pedal application causes the brake shoes to spread apart further than normal due to wear. These automatic adjustments help ensure the brakes remain effective and do not require the driver to manually adjust the brake shoes.