Geo,

If the overflow occurs after shutting off a hot engine, or after heavy
loads on the engine, the coolant could be boiling.  If that's the case, a
new pressure cap should solve the problem.  I'm on my third.

The pressure cap is the cap on the expansion tank, not the coolant overflow
tank.  It is on the left rear side of the Vanagon and has a hose connected
to it that goes to the overflow tank.

Mike  '84 GL   '60 Vespa    '58 Isetta