You wouldn't believe how many debates I've had over the years with people over the fact that you don't lose air (unless of course there's something wrong with your valve or pump). My favorite Pro Tip for those that want accuracy when checking their pressure: attach your pump, get the original reading (let's call it 180psi), then add in a few psi (your choice, but remember what pressure you're pumping to). Then unthread the pump until it "hisses" and reads zero, then thread it right back on. What's your pressure now? Do that 1-3 times until, when threading the pump...