I have a mountain bike with a front tire issue. It has what seems to be a very slow leak. I can pump it up and it will stay inflated for the whole day, but will be low again the next day. I then remembered having this problem once before and didn't know valve cores become loose and can be retightened until I had already taken the tire apart and couldn't find a leak and then someone told be about valve cores and gave me a valve cap with a tightener on it. So I used this tightener on my current tire by tightening it as much as I could, and then I pumped it up. The next day, it seemed to stay completely inflated, and I was confident that I had fixed the problem. Then the day after that, the tire was completely flat. So much for it being fixed!

The question now is: Does this still sound like a valve core problem? Is there anything that can be done with it other than trying to tighten it since that's already been done?

You can use spit on the end to see if you have bubbles coming out.
Else patch the tube.

I'll try the "spit test" tonight. If it is the valve core that's the problem, is there anything that can be done with it other than just trying to tighten it?

The valve core (assuming Schrader) is easily replaced. Think they're universal, so any local tire store should have one. Or just replace the tube, and while the tire is off, run a dryer sheet around the inside of the tire to see if there is a thorn or anything in the tire. Dryer sheets are great for this-they will snag on almost anything! Could be that if there is something in the tire/tube, when whatever it is, is positioned so it's on the floor, the pressure from being on the floor is acting like a "plug", allowing only a slow leak. When not on the floor, could allow more air to escape, as there is not pressure on it, so you lose the "plug" effect.

I've had mystery leaks like this before. In most cases it ends up being the tiniest pin hole that only opens up when the tube is manipulated in a certain way.

Sounds like over tightening. Just snug it up a bit past where it stops turning, that's all. Don't over tighten.

Sometimes they clog with sealant. The spit test or soapy water will reveal bubbles if it is leaking.

Valve cores are $10/dozen online. I just replace when they start to get clogged up.


-Tim-

Well, I was going to do the spit test on the valve last night, but it turned out to not be needed. I pumped up the tire to get some pressure in it, and then when I disconnected the pump, I heard the hissing of escaping air. I originally thought it was the valve core again and tried again to tighten it, but soon realized that the air wasn't coming out of the valve, but rather from a leak inside. It seems to be the valve stem itself. When I would twist the stem, the escaping air would either speed up or slow down depending on which way I twisted it. I guess this means having to replace the inner tube. A leak where the stem meets the tube can't be patched, can it?

One other question this leads me to is about how this could have happened. The only thing I can think of that could cause a leak between the valve stem and tube is when I pull the nozzle off of the stem after pumping up the tire. That's the only thing that puts a lot of pressure on it. Sometimes the nozzle is really hard to remove. Is there a way to fix that? I'm pretty sure that the current problem was simply made worse by my pump. Before, the leak took about 24 hours to make the tire low enough to need inflation. Now I pump it up and it only takes about 2 minutes to go completely flat. This change happened suddenly after I pulled the pump nozzle off the valve stem last night.

I've had a number of stem-tube failures, sometimes on relatively new tubes. Nothing to be done but to replace the tube.

I too have had a number of them fail. I bought a batch of tubes and had a rash of failures not on the stem but where the stem molds to the tube. No fix.
I think that sometimes it is an issue of how much flex is induced buy pumping a tire up with a hand pump. High pressure is hard to pump against and sometimes there is a lot of movement in the process.

BITD, tubes were fairly cheep, not today.

My bet too. I suspect that happens A LOT. Back when I used a full sized frame pump to fix my flats, I'd stabilize the valve stem and wheel against a tree or telephone pole while I aired my tire. Today I use a Road Morph pump because it has a hose to minimize valve stem flex during re-inflation.