> The valve is specifically designed to allow a small flow even when
> closed, to prevent coolant stagnating in the core.

FYI--I replaced the original front heater valve in my '87.  The
"ball" of the valve had a slight crease or valley in it, so it
always passed a little coolant when closed.

The replacement valve did not have that feature and closes 100%.
You'll have to get an explanation from a VW engineer about that
change.  Until then the debate about stagnating coolant can
continue....  ;-)

Richard