Steve, the overflow tube inside the neck was grinded down til it was flush with the inner wall of the neck to make sure the rubber disc of the cap can move up and down.
When 4 lbs are reached in the system the pressure presses the spring loaded rubber disc up and relases some water throu the tube.
This happened only once on a verry steep mountain road after I turned the engine off. Usualy I never loose eany water.
I got the cap from
Chevs of the 40s Frank
