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I think it's because it messes up the vacuum, the PCV is connected to that same valve cover so removing the cap creates a large vacuum leak.
BTW, that is not a good way to check for ring wear. Rings do not seal %100 of the compression so it doesn't matter how good they are you are going to get blowby. If they seal, say 96% and you have 135psi in all cylinders that still leaves you with about 6psi of blowby.
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