One of the guys checked his tires the other day before he left home and they were right on. When he got to the departure point a few miles from his house we noticed his front tire was almost flat. Checked for nails, none. Checked the cap on the tire valve stem and the "o" ring had become dislodged, became egg shaped and pushed the schrader valve open and he lost almost all of his air in his front tire. Problem was easy to fix and there's a gas station at the departure point. So, check the "o" ring to make sure it's properly seated before screwing it back onto the valve stem. I took the HD "o"ring caps off mine and screwed on a couple of nice knurled ones with a flat rubber gasket at the inside bottom of the cap.