I've noticed a slow leak from my rear tyre... about 8 lbs over a week (a bad valve I think). I had a spare 20mins today at work, so I thought I'd go and top it up (cos I carry the pump).
While there I figured I'd check the front. It was only down about 0.5 lb so I was pretty happy with that - even just getting the pump on/off loses a bit. Anyway, I took the pump nozzle-thingy (technical term) off the front valve, and swooosh - air just starts flowing freely out of the valve which had stuck in.
A bit of jiggling freed it up, so I pumped it again, with the air emptying again when pulling the nozzle off for the 2nd time. Luckily, I happen to carry a valve tool in the the panier, so I removed the valve. Not paying attention, the spring had it leaping over my shoulder in the carpark, then searching for it with my phone's torch. 15 minutes later I located it, cleaned it with spit, refitted, and the tyre pumped up without any problems.
I carry just about every tool with me on the bike, cos it has the space and cos I can.... except for spare valves. That will be fixed this arvo... assuming there is still air in them when I leave work.