Mavic makes rubber gaskets you can use.

I'm not clear on exactly how the presta valves work internally.

Does putting the pump on open the valve (by depressing the internal bolt), or does the air pressure do the trick?

This is the standard valve found on commuter bikes in Germany.
Also known as:
# Fahrradventil
# Blitz-Ventil
# deutsches Ventil
# Normalventil
# Dunlop valve

Pictured on my daily commuter with and without an adapter for use with standard auto pumps.

Pumps made for Presta valves also work on this valve.

+100

My memory, on-bike tool kit, and mechanical skills all improved markedly after my first few walks.

i've just got used to presta. it's fine, and i carry an adaptor. i'm still more comfortable with shrader, though.

Presta valves operate on air pressure. When you pump and the pressure in the pump exceeds the pressure in the tire then the valve opens and air flows into the tire. When you reach the end of the pump stroke, the pressure in the pump becomes less than what's in the tire, the air starts to flow out of the tire and, if all goes well, closes the valve. Sometimes the valve can get momentarily stuck in the open position. If the pump has a check valve, which most do nowadays, that takes over and there is no problem (at least until you detach the pump), but with an older type pump with no check valve you can get a surprise in the form of the pump handle striking back at you. Presta valves can also (and perhaps more commonly) get stuck in the closed position, which can be prevented by "blipping" the valve stem to let out a tiny amount of air before you start pumping.

Schrader valves are intended to be opened by having a pin in the pump head or hose depress the inner stem when the pump is attached. In this case a check valve in the pump is required. There's a spring in the valve to help keep it closed tight. I think there used to be valves with "softer" springs for use with pumps without a depressor pin, but I don't think I've ever seen a schrader pump without a depressor pin and check valve.

The spring in a Schrader valve is only really necessary with low pressures (as used in passenger car tires, or some wide offroad bicycle tires) and/or if a good valve cap is not installed. For many years I've used Schrader valves with the springs removed and pumps with the depressor pin removed. They then open and close by air pressure, the same as presta valves, but don't get stuck the way prestas do because the valve seat is flat rather than conical. I've even used a springless schrader valve on a high pressure low volume suspension air shock and had no problems with leakage.

Well, there is at least one person out there who thinks they have an adapter on their valve stem. But he's wrong. It's on top of my fridge. Hopefully he's got one on the other tire. It was a couple of months ago since I found it, so perhaps he's figured out by now that it fell off.

I prefer the presta , slower leaks between rides in my experience, that might be do to the tiny nut on the valve which you tighten after pumping it up, it will help keep the valve closed and air from leaking.
Also lower chance of getting crap in the valve if you loose your valve cover.
I don't carry an adapter anymore but i do carry a pump, so thats not much of a problem.

But you very rarely find them. Most bike shops only carry the rubber covered ones.

I will offend both camps, I have a presta on the front and a shraeder on the back. Here in ABQ, NM, USA we have these things called goat heads that are everywhere and constantly puncture tires, so everyone has to run slimed tubes. The goat heads look like this

http://farm2.static.flickr.com/1089/...bcadb8.jpg?v=0

I happened to have two slime tubes, one presta and one shraeder...

That really just seems like the worst of both worlds. Don't most mini-pumps require you to change out internal parts to switch between schrader and presta? Do you have any reason for doing this?

We have goatheads here as well. Yet a majority of the cyclists do not run slime tubes.

Most pumps require you to flip the rubber gasket in the pump head around to switch between presta and schrader. Well most that I have had.
That would be an irritating setup.

Schrader all the way. U can use the same pump for the bicycle/motorcycle/car, u can fill the tyres at every gas station (no need to carry fiddly, easily lost adapters) and u can borrow a pump from just about anyone, should the need rise. That Schrader is less reliable is a load of horsecack. Think about the centrifugal forces they endure on a 180 horsepower motorbike.

Flashback nightmares.

That valve has a replacement natural rubber element like a sleeve that one has to spit on it so it can fit.

The horror .... the horror (nah ... they worked ok, it just reminds me of cotter pins)

Or a car or anything else.

I used my floor pump to air up some car tires ... imagine that.

It took me less time than using the sub-standard pump at a gas station.

Both my bikes came with presta valves ... it does not matter ... each bike has its own frame pump; one pump per bike is the rule, presta or not.

I use continental gatorskins here in santa fe. No slime tubes yet, but it is not a bad idea either.

Nightmares based on ancient history or old wives tales, when was the last time you saw one? Modern Dunlop/woods valves do not use a replaceable rubber element.

In my understanding, presta valves do have a slower leak rate, and this seems to be related to the internal stem construction. I'm not an engineer. That being said, I ride schrader anyway :p

nightmares about cotter pins

the valves (dunlop i guess we called them) where ok ... held air in, let air in.

My pump does require the flipping of the gasket. It is probably going to be irritating, that is why I will eventually change the rear to presta, but the only slimed tubes that I had were 1 of each (I only have one schraeder valve tube, and it already had slime in it), and I wanted to ride last week, hehe! It is definitely not permanent, I just need to go to the LBS.

About the slimed tubes, I tried running non-slimed tubes, thick tubes, puncture tape, etc. and they all failed (in the summer riding on the bike paths here in certain areas I have had my tires literally covered in goat heads all the way around, I have to stick my foot onto the tires while I am still moving to scrape them off, because if I stop the slime will not work properly and I will lose all the air). I run 26x1.5 tires on my old XC racing crossmax wheels, and even with a ton of slime I have had flats (one time it was a giant hole caused by a nail or wire or something, the slime kept squirting out everywhere but the hole was too large to be plugged and I had to walk my bike home, another time I had a goat head in the tire and I parked my bike, the tire went flat, but after I came back from class I just removed the goat head, pumped the tire up, and spun the wheel which plugged the hole).

They are only less reliable in that they are more vulnerable to valve stem cuts. Also very few gas station pumps are capable of delivering the pressure needed for car tires at 35 psi much less the 40 to 60 psi for mountain bike type tires or the 90 to 120 psi for road bike tires. Additionally, seldom do I experience a flat close to a gas station. Most of the time I'm far enough away that a frame pump is far more convenient.

Finally, I'm not terribly happy to provide mooches with my pump because they are too lazy to carry their own. Just like a long walk can improve memory, a long walk will give you time to contemplate self sufficiency.

I also have a presta in front and a schrader in the back. The store only had presta when my rear tube-of-unknown-age was punctured, and I bought two. Then one went flat (pinhole leak) so I put the remaining schrader back in. Worse, all of my rims are drilled for schrader.

Inner tubes (or anything made of rubber for that matter) have slow leak rates due to the nature of the material. That leak rate is higher than any valve leakage...presta or shrader.

When using Presta valves on Schrader drilled rims, there is a rubber grommet than can be pulled over the Presta.

Note: If you unscrew your presta valve with it at the 12:00 position, as pictured, the air can all fall out.