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I told you it was a stupid question, and you will agree when I tell you how I solved the mystery. I put my mouth over the nipple (OK, go ahead and laugh now and get it out of your system), sealed it as well as I could with my lips and blew. I quickly removed the gauge and looked at the face and saw the needle returning to "0" having deflected in the positive direction. Likewise, I sucked a vacuum and again saw the needle returning from the negative direction.
All I can figure is that there are some VERY tiny holes drilled in the "plug" which permit pressure and vacuum to be seen by the sensing mechanism inside.
I know you are laughing ... and so am I.
--
Dave in Virginia
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