[kdavies3]

Pete, I disagree, I'm afraid.
The disc carrying the rubber washer (pressure cap), as you say is spring loaded.
This is variable depending on the application, but a common one is et to blow off any excess pressure in the system at 1.0 bar. Other than then the cap is sealed, otherwise air would potentially be drawn in to the sytem every time it cools down.

Kev Davies
Dax, 383ci Chevy Stroker, Tremec TKO
South Wales