I moved to the Vitorria Multiway valve stems and reduced the air leakage, substantially. There are VERY expensive, but worth it. They have the traditional cone shaped rubber stopper, which I find to be far superior to the stems with the hammer shaped stopper. I do give them a good tightening. I have them paired with Pirellis, a discontinued tire that was notorious for fast air leakage, and the stems have helped a lot.