When should brake shoes be replaced in an air brake system?

Brake shoes play a crucial role in the functioning of an air brake system, as they are the components that create friction against the brake drum to slow down or stop the vehicle. It's essential to monitor their wear to ensure safe driving conditions. The correct answer emphasizes the importance of replacing brake shoes when they are worn out or have reached the manufacturer’s specified thickness. This thickness is typically outlined in the vehicle’s maintenance manual and is designed to ensure that the brakes operate effectively.

Replacing brake shoes at the proper thickness prevents brake failure, which can lead to serious safety hazards on the road. If the brake shoes become too thin, they may not provide sufficient friction, leading to longer stopping distances and diminished braking performance. Regular inspection and adherence to the manufacturer's specifications help maintain the vehicle's braking system efficiently, ensuring driver and passenger safety.

While the other options touch on maintenance and performance aspects, they do not specifically address the critical factor of the actual thickness of the brake shoes as per the manufacturer's guidelines.