Review on 6 Inch Dually Valve Stem Extenders - 2 Pack: Heavy Duty, Easy Installation for RV Tires, Truck, Motorhome, and More! by Duane Kaul

Although they appear to be good carbide, a design flaw means they are either breathable or not breathable. A good valve stem extender will have a captive square O-ring. By capturing the o-ring in a double-sided groove inside the stem and using a square o-ring, the o-ring will not push out when the extension is tightened. This design is used in high quality aircraft such as optional TPMS sensors. These valve extenders have a loss (won't fall out, but won't get stuck either), an o-ring seal. Tightening the extension (even just by hand) can cause the o-ring to deform and loosen, either causing air to leak out or blocking the Schrader valve from working, preventing air passage. Unfortunately the only quality alternative is the Milton 4" extension which is too short for my RV twin valves. I sent back the ones that didn't work hoping the next pair would work better. (Warning: If the stem end of the original valve is rounded rather than flat, the o-ring is almost certainly not in place.)