I've had the same FOBO "T" valve stems for the last two sets of tires (25K-ish). Now, on the third set, and I'm now adding air maybe every couple of hundred miles. Curious to see how they do on the next tire change.
Check them with soapy water. Next time you have tires installed, give the locking nuts a little tweak on the inside of the wheel.
21 years Army (retired)
...been everywhere, seen everything, done almost everything.