Car tire

[olddavid]

Possibly a valve core that sealed properly last time you put air in.

I've had that happen before. After I check the pressure, I take a pocket-knife push the core in a bit and let it snap back out to make sure it's sealed. That seems to work.

 

[Texasmark]

Probably Nitrogen. Larger molecules than air and not nearly as heat sensitive as air.

That sounds more like it and probably is easily available in a high pressure bottle administered with gauges identifying the tank contents and the other one monitoring the outlet pressure, with delivery hose and nozzle. I remember seeing such when I was visiting.