What does the cylinder head check valve do?

[Oldmactech]

Doing a head gasket and one of the things I was told I need to do is replace the o ‘ ring on the check valve. Trouble is, even with a big slotted screw driver won’t budge. Even soaked with PB Blaster (probably not a good idea to put Blaster on rubber).

No budge. Once I get the head back on the valve is hidden forever, rather fix it now but I’m stuck.

Thanks.




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