The cooling system is designed to draw more coolant from the overflow
reservoir when it needs more, and discharge it back into the overflow when it
has too much. In hot weather, a properly operating system will
intake-discharge continuously.
I noticed this recently on a trip to Central Oregon where it was hot-whenever
I parked, the overflow reservoir was filling up. When the engine was started,
the flow reversed. The cap regulates the flow-when a certain pressure is
reached, the cap valve open, releasing pressure and coolant.
Pressurized systems do this to keep the pressure from getting to high and
blowing a hose or other component of the cooling system. All modern systems
are pressurized, which combined with the water/antifreeze mixture, raises the
boiling point of the coolant up to around 230+ -- degrees.

There is usually about 1 inch or two of air space when the cap is removed,
engine cold.
If you hear coolant gurgling into the overflow reservoir from the expansion
tank, don't remove the cap-excess hot coolant can spray out.
If you think you have a leak that is undetectable, have the system pressure
tested.
Unless you live in an area rumored to have fairies or elves, I don't think
you have to worry about the system being over-filled-it will purge out what
is in excess of it's need.
Robert
1982 Westfalia(I-4 Golf lll engine)

[text/html]