It's hard to tell from your photo, but it does look like you have some type of a one way flapper valve. the odd thing is i have never seen one like in your photo. Most of the one way pop it valves i have worked with have had a cage to contain both the coil spring and the check ball and most of the flapper valves have used a pig tail spring on the swinging gate unlike the coil spring in your photo.
This is probably some type of safety feature to prevent a complete loss of air in case of a major air leak in your trucks air brake system. Think of it as a duel system master cylinder like on your cars brake system. In the event that one line in your cars brake system were to get a hole in it the other independent side of the system work keep working, i think this is the same thing here.
