Ugh, I learned the hard way that you need to check tire pressure before every ride. I had never had a bike with Presta valves before this year and had no clue about how they work. After about a month with the new bike, I had two rides in a row on which I thought my legs were just made of lead. One ride was just two miles up to the store and back, and I seriously wondered what was wrong with me! Then I got the weekly RoadBikeRider e-newsletter, which serendipitously enough had an article about using Presta valves. Eureka! It wasn't my legs at all, it was that I hadn't pumped up the tires properly! Made all the difference in the world (and I'm probably lucky I didn't end up with a pinch flat). Now, if I were just smart enough to apply what I learned about low pressure and rolling resistance on my bike to my car tires, I'd be in really good shape...

Bruno, those Presta valves are a bit temperamental. I have a floor pump and I still end up having to remove and attach the valve head a few times to get it lined up properly. If it's not aligned you'll get that feeling that the pump won't budge, but it's definitely not because the tire's full.

run it, it seems that most pumps these days have valve heads for both Schrader and Presta valves. If yours doesn't, you can probably get a Presta attachment from the LBS. After removing the valve cap, there's a little bit of metal near the tip of the valve that unscrews. You should turn it to the point that it's at the tip of the valve stem. Then, gently push the tip of the stem in and listen for a release of air. Your valve stem is then primed and ready. Sheldon Brown, bless him, has pictures of this: Presta Valve Stem.