My theory is that these heads are leaking compression into the coolant from
the cracks that exsist between the valves.  My engine was running fine before
my head gasket blew.  When it blew I immediately shut off the engine and
pulled over and the temp gauge was showing below center (where it always
runs).
Yet when I sent my heads out to be pressure tested, the test showed that the
cracks between my valves (all Vanagon heads have these cracks) were going
into the the waterjacket.  So I had to go the new head way (these heads
aren't good cores).
I would encourage anyone who is replacing head gaskets to spend the extra $30
to get this test done by a competent local machine shop.  If they don't pass
the test, call me up and I'll send a new head out to you for $375 list
special.

Hope this helps,
Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242