Iveco 75E15 blown head gasket possibly ?

Hoping someone out there can shed some light on this issue.

I thought my head gasket on my truck had blown as Im getting a lot of air up through my expansion bottle, it looks like its boiling but its only warm to touch, so then lead me to believe its air or pressure.

A friend said this could be a one way valve within the water system that re-directs water to back to the expansion bottle if the water system gets too hot.

If anyone could advise on this issue or inform me where this valve is located, or possibilities to try to determine if its this 'said' valve or whether it is a blown head gasket I would be very appreciative.

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