I recently replaced the cooling exp tank and cap (black lower psi). You could hear vapor leaking out of the old cap which I attributed to warped threads on the tank. With the new gear, you can still hear gas escape, although not as much, even when the engine is cooling down. E.g., I can turn the motor off, let it cool and then take off the cap. If I put it back on, in a few minutes it will hiss, releasing vapor. So it doesn't seem like a bad head gasket would the be source. Is this the pressure release on the cap not working?

Thanks.
Paul
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1982 240 DL (265K) , 1996 850 GLT (125K)