I've tried my hand over the oil filler hole to check for excessive pressure. Felt fine. Today I thought I'd be a little more scientific
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Found a kitchen waste pipe connector and taped a nitrile glove to it. Blew up nicely by mouth. Made a rubber gasket to act as a seal when I held it down on the hole.
No inflation of the glove what do ever. However, when I revved the engine it started sucking the glove in.
I'm googling negative valve cover pressure and it seems to be a good thing. Can it be too good though?