Try some penetrant oil. Lightly tap with small hammer just enough to put some vibration to it. Tap around after you soak with oil. Don't force it too hard. Slowly work it so you don't damage it. ..
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After you get it apart, lube the o-rings with a silicone grease that is good to 500°F. That will keep it from do that again. The silicon connector di-electric grease should do.