All of the caps that I recall have a spring between the top of the cap and
the seal which mates with a flange inside of the filler opening. This flange
(seal) is below the overflow and prevents coolant overflow until the pressure
of the system, as calculated for temp., exceeds it's set point.
In order for the system to be bi-directional the spring must be a bimetallic
so that it will operate like a thermostat. All systems must be
bi-directional regardless of weather they were originally equipped with an
overflow tank or not. If they were not bi-directional there could be enough
vacuum created as to crush the thin walls of the radiator tubing.
Now I will grant you that I am sitting in isolation at 3:30 AM theorizing as
to the physics of what is happening but I am confident as to my conclusion.
FWIW
George
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