Bill,
I don't believe that there are any check valves in the air lines to the air bags. It is more likely a slow air leak in one of the fittings for the bag that is leaking down. I would spray soapy water at that bag and fitting, and then follow the air line to every fitting on that line back to the air pump.
Someone with more experience will probably chime in and may have better suggestions. Hopefully this will help you some.