What is the primary purpose of the supply line in an air brake system?

The primary purpose of the supply line in an air brake system is to transfer compressed air from the compressor to the air tanks. This function is crucial because the air tanks store the compressed air needed to operate the braking system effectively. The compressed air is what allows the brakes to function, as it engages the brake chambers to apply the brakes upon driver input.

In an air brake system, it is vital that the compressor continuously replenishes the air in the tanks, ensuring there is always sufficient pressure available for braking. Without a properly functioning supply line, the air tanks would not receive the necessary compressed air, leading to potential brake failure or inadequate braking performance.

Other options relate to functions that are not associated with the supply line. For example, returning air to the atmosphere would involve different components of the system, while the connection between the brake pedal and brakes is typically managed by control lines, which are separate from the supply line. Backup air pressure is managed through dedicated reserve components in the system rather than through the supply line, emphasizing its distinct role in transporting air from the compressor.