During air-tank drain valve testing, what indicates that the drain valve is functioning properly?

The indication that the drain valve is functioning properly is when it allows air to escape without obstruction. This is crucial because the primary purpose of the drain valve is to release excessive moisture and compressed air from the air supply system. When the valve functions correctly, it should permit the escape of air freely, which indicates that there are no blockages or malfunctions hindering its performance.

While releasing pressure consistently and draining water are also important aspects of the valve's function, they do not encompass the full scope of what a properly functioning drain valve should do. A valve that opens and closes smoothly is certainly a good operational characteristic but does not alone confirm that it is performing its essential function effectively. The critical measure of proper functionality lies in its ability to allow air to escape without any obstruction, ensuring the efficiency of the air system and preventing moisture build-up.