Chris,

I am not sure that I understand your use of the term "overflow tank."

My definitions:

1) Expansion tank:  The coolant tank in the left rear of the
   engine compartment.  It has a complex cap with a fitting for a hose
   that leads to the coolant recovery tank.

2) Coolant recovery tank: The tank in the center rear of the engine
   compartment.  It's cap is a simple screw on type.

The expansion tank cap has two valves.  One valve releases pressure (and
coolant) into the attached hose when the pressure exceeds some level
(around 15 lbs/in*in).  The other valve opens to the attached hose when the
pressure is less than atmospheric to allow coolant to flow back into the
system from the overflow tank as the engine cools.

If you are positive that the expansion tank cap is working correctly, then
something in the cooling system is causing pressure to constantly build,
such as a leaky head gasket.  However, if everything else is OK, you have
the symptom of a bad expansion tank cap.

Mike   '84 GL    '58 Isetta