So I had purchased a bicycle trailer, and it showed up today. Needed assembly, of course. I put it together, and pump up the tires to the recommended psi.
Problem - right tire is leaking air from the valve. First thought through my mind is that I have a defective tube/valve in the tire.
Next thought through my mind: "Huh. I wonder if it's just the valve stem that's loose? Did I keep the valve stem tool?"
Yes I did indeed keep the valve stem tool. And yes indeed it was just a loose valve stem. A couple of turns to the right, and it stopped leaking air.
And that's why I keep that tool. Well, that, and the one I have stashed in my car so that I can remove the valve stems of the various assholes who park like they own the entire parking lot.