What is the main purpose of relay valves in a rear air brake system?

Answer: B — To minimize delay of brake application on long wheel-based applications

The main purpose of relay valves in a rear air brake system is to minimize the delay of brake application on long wheel-based applications. Relay valves play a crucial role in ensuring that the air pressure generated by the brake pedal is effectively transmitted to the brake chambers on the wheels, particularly in vehicles with longer wheelbases. This is essential because, in longer vehicles, there can be a notable lag between the brake pedal being pressed and the actual brakes engaging at the wheels, which could lead to less effective braking and increased stopping distances. By utilizing relay valves, the air pressure is more rapidly directed to the brake chambers, allowing for a quicker response and a more immediate braking action. This design helps to synchronize the braking across all wheels, promoting better stability and control in braking situations, especially at higher speeds or when carrying heavy loads. This improved responsiveness is paramount for maintaining safety and performance standards in larger vehicles such as buses and trucks.