Guest

Yesterday, my front tire blew when I rode over something, so I changed the inner tube, pumped it up to about 80 psi and rode the rest of my ride. When I got home, I pumped up my tire to the recommended max- 120 psi (I usually pump it up to 110). When I went to screw on the cap, I had the oddest problem- I'd screwed the inner tube valve shut, but when I tried to screw in the cap, my tire immediately started letting out air. I thought I'd done something wrong, so I checked the valve again- yep, I'd screwed the valve closed. I tightened it just to be sure. I tried to put the cap on again, but the air started coming out again. So I let out the air completely and repumped the air back into the tire. I pumped up to 120 psi again, and I screwed the valve shut again. But when I went to put on the cap, I get the same problem- the air is hissing out. So I repump the air one more time, and this time, I just loosely put the cap on the valve and turn it about half a turn, just to keep the valve from being exposed.

It's been a day, and the air hasn't let out of the tire at all. But if I try to screw the cap onto the valve, it starts letting all the air out. Am I doing something wrong, or do I just have a defective valve here?

:confused:

Koffee

Guest

Oh, P.S.- I have a Pyramid Bicycle Tube (model 6376) presta valve 700x23/25C.

Aggressor

May also be a defective cap, madam Koffee

Guest

No, the cap is fine. Actually, I tried 2 caps. Neither of them seemed to work.

moabrider47

Koffee,

If you push on the end of the valve (with your finger) that is visible when the small nut is screwed down all of the way, does the tire let air out? If so, the only thing that I could think of would be a defective valve. You don't have to have the valve caps on for riding, so as long as the tube holds air without the cap, I would just ride it until the tube finally goes or you decide to replace it.

-Moab

DieselDan

You don't really need to use the caps.

supcom

Some presta valves have an inner core that screws into the valve stem. The inner core can loosen and start backing out. If this happens, you will lose air between the threads of the core and the stem.

The solution is to get a small adjustable wrench and tighten down the inner core. There are two flats on the inner core at the top of the valve (just below the small nut that locks the valve plunger in place. Gently tighten this and the leak stops.

It could that putting on the valve cap is pushing on the inner core just enough to allow air to escape so it doesn't seem to leak without the cap in place.

khuon

The other possibility is that the threads for the valve are mucked up allowing you to only tighten it to a certain point but not enough to close it completely.

fubar5

Caps? What are caps?


nevermind, those are the little plastic thingies I always throw out..


Seriously, the caps are pretty much irrelevant.

Ed Holland

Very strange! Supcom and Khuon probably have the answer. However. Also I once accidentally bent the centre pin on a presta tube whilst pumping and it refused to seat & seal properly after this. It might be worth a closer look.

It's juts my opinion - I don't mean to hijack this thread and start a war..... but

The plastic caps are not irrelevant if you ever ride in mucky conditions. In rain and especially during winter when there is grit-salt on the roads a lot of dirt gets thrown at the bike and especially the rear wheel. I much prefer the idea that something is keeping the valve clean. (end of lecture )

Good luck,

Ed

lotek 

I concur with Ed. If the valve stem is bent (even slightly)
when you screw the cap on it could be engaging the
valve, no cap no leak. If you can replace the valve core
otherwise new tube time.

Marty

Guest

Ok, checked and the valve stem is bent... I must have done it at some point when I was merrily pumping along in my haste to get inside to that bath- oh well!

I will ride with this tube for a bit, and then change it when I hit my next flat, but not worry about it until then.

Thanks for the suggestions!

Koffee