I DIS-agree with the statement right below (by John
Rodgers)

> If the system is operating correctly, as the coolant
> expands when it is rising to operating temperature,
> coolant will flow from the expansion tank into the
> overflow tank. That is proper.

In my 1991 (syncro Westy) when the engine heats up to
"normal" operating temp, the system is pressurized! but
the coolant cap does NOT (and SHOULD not) open (to
relieve that pressure) till the van begins to overhead! then
the cap lets go and the 2nd expansion tank takes the
excess.  The van's coolant system is designed to run
pressurized! to lower the boiling point.

As you may know, last year I had my van in Egypt and
even at 44 degrees C (outside) the system did not
overhead - only when we were in the Cairo on a hot day,
with a hot engine, in a traffic jam, would the pressure rise
enough for the cap to let go into the 2nd expansion tank.

I may be wrong, but I do not think so.  Malcolm