Positive pressure at air box?

OK after I performed my last service I started her up and while leaning over the air box I noticed what seemed to be puffing from the seal area of the lid and box. I removed it and the seal was perished and had a section missing so off I went to Clark rubber and made a new gasket. I had already replaced it when I realised I should have confirmed it was puffing out and not just sucking and sucking less if that makes sense. The only way I could see it being positive pressure at the air cleaner was from the crank case but how much is there at idle? There was no sign of oil residue on the filter side of the tee in and I have no catch can installed yet. Any ideas fellas?

The crankcase couldn't pressurise the intake. It would have to pump more air through the breather than the engine was drawing in.
The only way you could pressurise the intake is if an inlet valve is open long enough for the piston to force air back up the intake. This could either be from incorrect valve clearance, cam timing out or a valve not seating. Either way it would be running rough especially at idle.

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