Malcolm,

Thanks for the great explanation of how the coolant tank cap works.  As far as
preventive measures to minimize the chances of bad things happening to my
engine, would you agree that the following are the measures  that mortals can
take:

1)  Change coolant regularly (once per year) using the VW blue stuff.

2)  Test the cap for outgoing functionality by blowing on it, hopefully to sound
like a duck call.  To test for its ability to let coolant back in, I guess the
best idea is to observe if this happens by looking at the expansion tank level
when the engine is hot and then compare it to when the engine is cold.

3)  Monitor the coolant temp. indicator on the instrument panel.  Although I
wonder if the placement of the coolant sensor is such that it would give you an
early warining that parts of your motor are too hot.  Or is it telling all is
well while your heads fry.

4)  Bleed the system thoroughly, by raising the front of the vehicle and opening
the bleed values, as Bentley states, to get the bubbles out.

5)  Drain the coolant out for prolonged periods of inactivity, to minimize
corrosion.

6)  Keep your fingers crossed and say your prayers before bedtime.

Anything else that you can think of...

Charles