ks1111

ok, this is driving me nuts. Wanted to inflate my tyre and now ended up flat and no clue how to resolve it.

Took out presta valve which seems to work fine, I can blow air into it when the top is unscrewed and pressed down and the air is blocked when the core is pushed up - fine.

Took out valve extender, its just a hollow tube for air to flow through.

Screw the valve onto the extender and suddenly hardly any air flows through. nothing has changed in this set up since I was able to pump air into it before, somehow the internal screw threads inside the valve extender must be restricting the air?? I cant understand it

anyone had this weird issue. Im just testing a valve extender with a presta valve screwed on on one side, the other side is open nothing attached.

thank you

Iride01 

You unscrewed the little nut on the valve all the way didn't you? Sometimes the valves get sticky and it's hard to get enough pressure with some pumps to break that seal. But most of us burp the valve before putting on the air chuck. That way we know the valve opens.

There are some differences between some brands of extenders. Might help to know what brand and model your extender is.

Welcome to BF.

DaveSSS 

I'd try to avoid extenders. Use long stem tubes or if tubeless, stems long enough to work without extenders.

ks1111

ok i think this could be happening. When I pump air through very very slowly it flows (too low pressure to be useful). If I pump at normal speed and high pressure it seems like the air doesn't flow into the small gaps through the valve but instead the back pressure builds up instantly and pushes the valve stem upwards sealing it off! I find this a bit bizarre it cant be designed like this. Does this mean my valve is broken in some way?

Iride01 

I'd think there is something wrong with your valve. Either you aren't opening it all the way or it's got some gunk of some sort in it. However anytime there is back pressure coming from the tire/tube side of the valve, it will seal itself off. That's part of the design. There are no springs to lift the presta valve to it's closed position.

Do you have another valve core to try?

ks1111

thanks. Did some more testing. The valve works fine on its own and fully open i can blow air through it freely. only when its screwed into the extender the issue occurs.

When i look at the extender there seems to be a narrow constriction just when the tip of the valve end touches it when screwed in (is this normal?), I think this narrowing is somehow constricting the airflow creating the back pressure.

unfortunealty i only have this one extender and one valve. Its really puzzling me.

cyclezen

From what you describe, as prior noted, the valve stem may be sticky/tight. Try working the valve stem a bit (when screwed open) to loosen it up some...
And the combination of a 'tight' internal diameter of the valve extender and tight valve stem, could restrict flow greatly.
Then if the valve extender is smooth where the pump head fits over it - the back pressure build up can be strong enough to blow the pump head off. I experienced that when first going to TPU tubes which had a smooth valve stem where the pump head fits - asked here, and a response mentioned sand-papering the plastic valve around the valve stem to add some friction/grip. Worked a charm !
If valve extender internal diameter is too narrow to allow good air flow, not sure how best to handle that... New Extender? or maybe try to use some small girt sandpaper and circular sand interior being careful to not sand the threading ? If you try any of this, let us know how that works...
Ride On
Yuri

Iride01 

Maybe that extender wants a specific make of presta valve. I think that like Schrader valve cores, presta valve cores have differing lengths and other slight differences that in most applications don't make a difference. But for yours maybe it does.

What brand and model extender do you have..

Troul 

I've ran into an issue with presta stem extenders. The valve was not the same fit form for function with the joining fixed stem & wouldn't seal. IIRC the threads had some sorta issue, might have been too short to fully seal. I didnt spend much time fussing with it & just returned the extenders & bought the length I needed. It all stemmed from the LBS not having the stuff I was after resulting in the unfavorable experience with an online order using the poopy extenders.