What is the primary reason for locking glad hands when not towing?

The primary reason for locking glad hands when not towing is to protect against contamination. Glad hands are used to connect the air brake lines between the tractor and trailer, and when they are not connected, the open ends can be exposed to dirt, moisture, and other contaminants. These foreign materials can enter the air system, potentially leading to blockages or malfunctions within the air brake system. By securing the glad hands when the vehicle is not in use, you prevent these issues from arising, ensuring that the system remains clean and operational for safe driving when needed.

While saving fuel and ensuring safety are important considerations, they are not the main focus in this context. Preventing air leaks is also a valid concern, but the most immediate risk when not towing involves the introduction of contaminants into the air system through the open glad hands.