David,

You said: "Got to get rid of the root cause - leaking head gasket."

I only was suggesting to look before leaping.  A head gasket leak could 
easily allow air into the system during the cool down and defeat the purpose 
of the recovery valve.  However, air in the cooling system could also be 
causes by the malfunction of this valve or any other leak under negative 
pressure besides a head problem.

Unfortunately, the history on the Vanagon suggests that leaky heads are more 
likely than bad coolant recovery valves.  But, I'd still look.  And, you did 
too.

Have a nice day!
Mike