My Greyhawk had multiple leaking problems with the extensions on inside dual tires. I drove it to a good tire shop where the guy replaced the old valve stem + extension with a new metal valve stem (about 7” long and bent to extend into the outside wheel). That stopped the leaks and it is easy to check & add air. I think it was about $50/tire for labor and the new valve stem, definitely worth it.
I also purchased the Truck Systems Technologies (TST) TPMS and love it.