Automatic drain valves mounted on main reservoirs must always be positioned for what type of operation?

The correct choice is automatic, as automatic drain valves are designed to operate without the need for manual intervention. Their primary function is to expel moisture and contaminants from the main reservoir, which is crucial for maintaining the air system's efficiency and preventing issues such as freezing or corrosion. This operation ensures that the air supply remains clean and consistent, allowing for optimal brake performance.

When positioned for automatic operation, these valves open at predetermined intervals or in response to specific conditions, ensuring that any accumulating moisture is effectively managed. Choosing manual, emergency, or maintenance positions would not allow the valve to serve its intended function continuously and could lead to issues in the air brake system or reduced performance when it is needed most.