MattyA

This is probably a basic question but I have searched a bit and cannot find a great answer.

This is my first set of tires with Presta valves, and I think I am using them incorrectly.

After I pump up a tire using a floor pump with a presta valve I seem to get a lot of air escaping when I remove the chuck from the valve. I experimented by pumping it up to 120, removing the chuck (and hearing the "woosh" of air when I pulled it off), then re-attaching the chuck. The reading was almost 105. So I lost 15 psi from removing the chuck? Is that normal? Is the chuck broken (brand new Specialized Airtool Pro)? Am I doing something wrong (likely!!)? When the guy in the LBS showed me how to use the pump he got the same air escaping sound when he pulled the chuck off, so I assume it is normal.

Thanks to anyone who can help me here...

Matt

Retro Grouch 

You worry too much.

A Presta valve is a simple poppet valve. As long as the air in the inner tube exceeds the outside air, the valve is pushed closed. When you pull the chuck off of the inner tube, the air pressure in the pump hose drops and the presta valve snaps closed. Now you still have high pressure air in your pump hose with nothing to hold it in so the "woosh" that you hear is coming from your pump, not your tire.

So now you put the hose back onto your tire. Assuming that the process of replacing your pump chuck forced the presta valve open, the tire air would have to fill your pump hose before the gauge would register any pressure. Since you let some air out of your tire of course you are going to get a lower pressure reading.

MattyA

Ah, that makes a lot of sense.
Thanks much!

Panthers007

When you remove the chuck, screw the little brass top down on the Presta-Valve. This shuts it off to losing pressure from bumps, etc. And try to remember to un-screw the brass top before putting the chuck on the valve.

Riverside_Guy

My experience is that the most minor of deflection to the side of the "lock nut on threaded rod (what should this be called?) will open the valve and air will escape. Pulling the chuck off could cause this, no? I DO get a very short bit of air escape when I pull a pressure gage off... AND I'm being very careful to pull it off as straight as I can.

probe1957

Use a pressure gauge to check the air pressure in your tires. Use the pressure gauge on your pump only when you are inflating your tires. It doesn't take much to let 15 psi out of a bike tire.

norskagent

I am not fond of the flip lever / shrader-presta valve hose ends found on most floor pumps now. I cut mine off and put a dedicated brass silca presta valve fitting on. I used a small hose clamp from the hardware store to secure it. It pulls right off with little air loss. It's easier to pull off the smooth presta valves, sometimes pulling it off threaded presta valves is a struggle.

noglider 

Proofide, you're kidding, right?

noglider 

OK then. You're a funny guy, even if you aren't joking.

The pumps I had that looked like that worked poorly. The Zefal HP was a breakthrough when it came out. Now I usually use a floor pump with built-in gauge. It takes very little effort to get to 120 psi.

Long ago, I took a three-month tour on bike through France and the British Isles (so I now know what twenty-five bob means). The valve pin broke off one one of my tires. I was using tubulars, like the idiot I was. Since I couldn't measure the pressure with my guage, I just added a little air every day, to be safe. Well, I was adding more than I was losing, so eventually, the tire blew out from overinflation. Awesome: how much pressure does a tubular need to blow out? A lot, I think.

due ruote 

One other thing you should be aware of, although I doubt this is what happened to you. On some (but I don't believe all) presta valves, it is possible to unscrew the entire valve core from the stem. I have had the core come slightly loose - enough to cause the sort of slow leak that emptied the tire overnight.

It's a good thing to check before yanking the tire off the rim, or worse, making an incision on a tubular.

bkaapcke

If your rims aren't too deep a Vee, get schraders. Drill your stem holes to 11/32". bk

Al1943

The real beauty of a presta valve is that the hole in the rim is smaller for more strength.

bkaapcke

Big increase in strength there? Don't think so. bk