Gary, as you may already know- you can confirm the combustion presure/gas in
the coolant with a "block tester". You can get these from your local NAPA for
about $50.

You are describing precisely the symptoms which I experienced with my van
last fall before putting it away for winter.  Mine ran fine around town but
when the RPMs and load were up as on the freeway it would belch all it's
coolant out.  What a mess.  I finally got the courage up this spring to try
to do the head reseal job myself.  Although it is not a quick job it is also
not that difficult given the available info in the Vanagon.com archives and
the experience of your fellow listees.  I brought my heads to a local engine
machine shop and had them cleaned and tested for $35.  Mine tested fine.
Like yours they were relatively new - only 25K miles since they were replaced
new by a VW dealer.  Sorry,  I cannot comment on your sealant options... I
would stick to the VW sealer products for the job as has been recommended by
very experienced folks like Bob Donalds.

It is my opinion that you not try to simply re-torque the cap nut as you have
already set up the leak routing and this remedy would likely only last until
you were on a trip if at all.  Also,  I have seen some convincing material in
the archives which indicates that the combustion gasses in the cooling system
will cause a chemical deterioration of components (experts might comment
further on this point).  From this it is recommended that the cooling system
be flushed regardless of the remedy.

Good luck,  I hope this was useful.
Eric Small
So.Portland, Maine
'86 GL Weekender