Forest, it could be the extension BUT if the tire pressure sensor is touching any part of the wheel, it may be the valve stem on the inner tire itself leaking. These tire use screw in valve stems. The valve stem is placed through the wheel, then a nut is put on the inside of the wheel and tightened to compress the rubber. As the rubber ages and/or the nut loosens from wiggling the valve stem, you can get a slow air leak around the valve stem.
I would check with a heavy truck tire shop to see if it is the valve stem or the extension that is leaking.
The other thing to check is to make sure the valve is securely seated in the stem. You can get a valve stem remover from an automotive parts place or even Wal-Mart. Make sure the valve stem is tight (clockwise to tighten).
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