Coolant filler cap seals

Thinking through, I'm not convinced the coolant filler cap is doing its job.

Already confirmed - by pinching the top lugs on the cap it will hold 20psi positive pressure (when cold) and I don't have any leaks anywhere on the coolant system.

The cooling is behaving itself at this point - as per the first 4,500 miles - maintaining engine temp but finding a coolant level approx. at the bottom of the water rail.

Issue

I don't believe the cap bottom seal is pressed hard against the shelf/lower casting edge; thus not effectively holding the pressure the cap is rated at, my guess its not holding any pressure hence losing coolant out the overflow just through circulation and the resulting low settle point.

- Cap top seal lets the system suck back coolant from the overflow when cooling down.

- Cap bottom/sprung seal holds pressure in the system before releasing at pre-determined 20psi and allowing overflow.

Experiment

Pending finding a long reach cap, I made a gasket cut from 1-2mm rubber which sits under the bottom gasket while still allowing free movement of the suck back disc. (I was going to sit the ring on the casting but tucking it under the cap gasket is a better idea from Foxtransit on GBSOwnersclub):

Now installing the cap requires 1-2mm downward pressure, depressing the internal spring and pushing the inner seal against the 'shelf' inside the casting, followed by the normal extra pressure to fully rotate the cap into its locked position: (thats fluorescent antifreeze appearing opaque due to the camera flash, not rust, in the coolant)

Testing may have to wait for the weekend, November seems to have turned up with rain and cold this end of the country!