The factory valve stem is the tire pressure sensor.
When you get HRE's you need to strap the factory sensor to the barrel of the wheel in order to retain the systems functionality.....
Anyway the straps break. I had one break (the sensor bounced around inside the wheel) and another one cracked. This second time around I applied some RTV on each side of the sensor to help hold it in place.
As for you leak, I think you're on the right track with the valve stem.