If You have air in the expansion tank, the tank cap doesn't work. If the water volume extends, the air on the top of the expansion tank will be pressed thru the cap and the small hose into the refill tank. While cooling down, water is sucked in reverse direction. In this way, the expansion tank must always be total filled.
If not, cap must be replaced and/or the cooling system has a leak, where air is sucked while cooling down.

I had these problems to and vw wasn't ablto fix it. The discribtion of the tank caps function is wrong in Bentley.

Conrad
86GL
-----Urspr�ngliche Nachricht-----
Von:    Bryan Feddish [SMTP:bfeddish@NETREACH.NET]
Gesendet am:    Dienstag, 4. August 1998 18:50
An:     Vanagon@VANAGON.COM
Betreff:        Re: How hot is HOT?

Drive the van around the next day and get it good and hot. Let it cool
overnight again.  There will be more air in there.  Replace it again.  Keep
doing this, mine took a week, every day until no more air is in that tank.
BTW, this will make sure your heater works properly in the Winter too.
        In addition, I have one of those fan switches that goes on a few degrees
earlier that I got from Kyle.  Actually I rarely hear the fan kick on except
after I shut the engine off.

Bryan