Presta Valves keep failing New to this site. Hope you can Help. I have been commuting on a hybrid for years. A nice CUBE with hybrid tyres with Scrader valve. The odd puncture here and there. Par for the course.

Now I have finally upgraded to a Willier Izoard, with race tyres and Presta vlaves.

First couple of trial rides no problem. Then this weekend I went to go out and realised that the tyres were fairly flat. I hadnt ridden this bike for around a month so I figures the pressure had just dropped. 

So I topped up with air. Back wheel was fine took to 110PSI (Max 120). Did the same on the front wheel and went out riding. Within 10 mins, I noticed front tyre was completely flat. It seems the valve failed as I could pump the tyre back up but then as soon as it relased the air just shot out of the valve!

I had a spare tube with me but no pump or tyre spanners. Luckily, I bumped into a very nice Bulgarian chap who was preparing all his kit for a two month cycling holiday and had all his gear literally out in the street. This guy really knew his stuff and within minutes he had changed the inner for me (and way quicker than I could have done it by the way" Had the tyre pumped up (to 100 PSI this time as thats all his guage went to) and off I went.

Within another 10 mins the tyre was completely flat again. Once more it appeared as if the issue was wth teh Presta valve, and this time it would take no air in either. 

I find it hard to believe that I would get two complete valve failures in the space of ten minutes on two seperate tubes, especially when it was replaced by an expert!! 

Does anyone have any ideas what maybe goign wrong here? I am a bit of a Noobie to race tyres so would appreciate any suggestions? It is a brand new bike so I doubt it could be the rims, and I checked tyres for holes pinches tc, but it does seem to be the valve. Werid!
Presta valves are fairly fragile things. It's easy to bend the stem once it's unscrewed. They work by the tube pressure being higher than the atmospheric pressure. This causes the valve to be forced closed. It is not until you either loosen (unscrew) the valve and depress the stem or place a pump on it and increase the pressure above the valve until it is overwhelmed and allows air in.

Point is, you can leave the valve "untightened" and air won't escape. The valve mechanism failed because for some reason it did not return to a closed position completely and leaked out. Then when you tried to top off the tire again it was further stuck Two best guesses:

1) You had a run of bad tubes. Welcome to road cycling. Things are more fragile. This is my first choice. Buy a different brand and see what happens.

2) Schrader valves are more durable and forgiving of rougher treatment. Perhaps you were use to them and when you topped off the presta tubes you bent the valve core and it wouldn't fully return to sealed. 

Don't give up. I've had bad tubes before.

And one last thing: If you are a valve bender (like myself) I found the cure. Silca and SKS both make a chuck that doesn't need to make contact with the valve core. It uses the pressure from the pump to overcome the resistance of the valve core. Here is the SKS version, currently on sale at Saint John Cycles. You just push it on and pump. Couldn't be easier or more effective. Good only for presta though.