What is done when replacing the double check valve between the primary and secondary air tanks?

When replacing the double check valve between the primary and secondary air tanks, it is essential to drain the air system. Draining the air system is critical for safety and to prevent any air pressure-related mishaps during the replacement process. This ensures that the airflow is completely stopped and allows for a safe and effective replacement of the valve.

By doing this, you minimize the risk of accidentally releasing pressurized air when disconnecting the valve, which could lead to injury or damage to the vehicle's pneumatic system. It also helps to avoid contamination of the new valve or the air lines.

In the context of replacing components in an air system, maintaining safety and ensuring the integrity of the system is paramount. Hence, draining the air system is the proper procedure to follow when making such repairs or replacements.