If gas is seeping from the up-side-down cylinder head combustion chamber you have a problem with the seat, the valve or both. I would think the gas at the top end of the valve spring is just the thin liquid following a path past the valve head and continuing down the stem. Valve guide to valve stem clearance may be small but's not zero. With the valve out to deal with it's problem, put it mostly back in and give it a wiggle.-If you can't feel side to side movement it's OK. If it does have play it wont be the only one. But valve guide play will result in oil consumption--not loss of compression. - Dave
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