Before you can put air in a Presta valve, you must unscrew its tip. Look closely and you'll see that the valve's tip is knurled to make it easy to turn it by hand. Unscrew it all the way (counter-clockwise) and then press the tip down until some air escapes. This is important because it frees the valve, which usually sticks after being sealed for a while. Until you free it, it can be difficult to impossible to put air into the valve.
Here are six other valuable valve facts:
1. Replaceable Presta Cores
Some Presta valves have replaceable cores. You can tell if yours is by looking for wrench flats on the sides of the valve just below the tip. A replaceable core is a nice thing if yours gets damaged somehow. However, it's also something to check regularly because if it loosens, you'll develop a slow leak and get flats all the time. The solution is simple, just snug the valve by tightening it with an adjustable wrench (turn clockwise).
2. CO2 Cautions
Take extra precautions using CO2 inflators on valves with replaceable cores. The drastic pressure drop as the CO2 leaves the cartridge super-cools the cartridge and adapter. In damp weather, this can freeze the cartridge to the valve. And when you unscrew the adapter, you extract the valve core with it, deflating the tube. To prevent this, after inflation, squirt the valve with some water from your waterbottle to de-ice things and then carefully remove the adapter from the core.
3. Converting Schrader Holes For Presta Valves
If you use a Presta valve tube in a rim drilled for Schrader valves, you'll notice that the hole is too big. This isn't a problem unless you ride with low air pressures as some off-ride cyclists like to. In that case, the Presta valve may creep as the tube shifts inside the tire. This can lead to a bent or broken valve over time. To prevent this miscue, install rim grommets, O-ring-like rubber washers that fit in the valve hole reducing its diameter to match the Presta's.
4. Protect The Valve When Pumping
An important tip about valves is that they're not indestructible and they're at the most risk when you're pumping up the tire using your frame-mounted pump. To protect the valve, always support it by holding the end of the pump that's on the valve in such a way that you can hook a thumb or finger over the tire. That way, as you push to inflate the tire, you're pushing against your hand and not the valve, which will bend or break if you push against it alone.
5. Valve Nuts
A common question with Presta valves is whether or not it's important to install the valve nuts (knurled metal rings that are used with threaded valves). Not all Presta valves are threaded from top to bottom. But, if yours are threaded, there's a good chance that there are valve nuts on them. These can make it easier to inflate the tire because they hold the valve proud of the rim making it easier to get the pump head on them. Be sure not to tighten them too much, however, or they'll be difficult to remove by hand when you have to fix a flat on the road or trail. Overtightening the nut can also put pressure on the valve/tube junction where it passes through the rim and cause a flat.
6. Dealing With Slow Leaks
One final tip: when you're searching for a slow leak, don't ignore the valve. Sometimes valves fail and air seeps out. To check, put a little spit on the end of the valve and stare at it for a few seconds. If the valve is leaking, a bubble will form. Often you can tighten the valve and the leaking will cease. If this doesn't do the trick on a Schrader valve, try removing the valve core, putting a drop of oil on the spring and reinstalling the core. This will usually stop the leak. To work on Schrader valves you'll need a valve cap with a built-in valve tool or a separate valve tool. These have pronged ends that fit inside the valve to grip and turn the core. They're available at auto-parts stores.