Proper diagnosis is a must. If there is high pressure in the cooling system then its got to be coming from exhaust gases. Nothing else pressureizes. I think some people will have a tester to verify the presense of exhaust gases in coolant. But as to why its getting in there, there is something either not sealing between the head and block OR the head is internally damaged (cracked).

Has the head ever been pressure tested?

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Brian Toscano