Why do my inner tubes keep developing leaks through the valve?
 

If they have a removable valve core can you tighten up the valve core? It helps to have a valve core tool for this or you can use parallel jaw pliers or small vise grips for this.

From your post it sounds like leakage around the valve core. Did I understand you correctly?

It is a leakage through the valve which I assume is called the core. Why would it start leaking now after I repaired it but not before, when I was on the road?

Maybe it moved slightly when pumping air into the tube.

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https://www.aliexpress.com/item/1005003156480961.html

You might try better tubes. I use to have a lot of tube failures when I was buying the cheap bike-store brands (like Q-Tube). Since I switched to Schwalbe and Continental, I haven't had a bit of trouble with defective tubes. You really do get what you pay for in bicycle tubes.

You could well be right. I'll try some better ones.

Assuming Presta:
Because or reasons, I use the valve caps. And occasionally these can be on tight enough to loosen a removable valve core.
If that happens, IME, the core will usually continue to back out entirely. But I suppose it’s possible that it’ll simply back out a little bit, enough to leak.
Or maybe you’ve forgotten to set the knurled locknut?
That too can give an unpredictable result.

It's a Schrader valve. I'll try better quality tubes.

is it leaking through the top of the valve or from a cut at the bottom of the stem? Although this is a Presta valve, the parts are analogous to Schrader valves although the Schrader valve is clad in rubber. The green arrow points to the core. Core leakage is very rare on either Presta or Schrader. It is also something that is easily fixed.

The red arrow points to the stem. Stems being cut during inflation is very common problem and is due to user error the vast majority of the time. You can cut the stem if the rim has a burr but those are rare. More often, the user pulls on the stem while pumping and cuts the rubber around the bottom of the stem on a Schrader valve. If you are using a hand pump that you pump at 90° to the stem, you are much more likely to cut the stem than if you are using a pump with a hose where the movement of pumping is directed into the ground. If your problem is due to cut stems (more likely than a core failure), you want to reassess the way your pump the tire while on the road.
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It could be bad tubes but as others mentioned it could be a loose valve or the valve was damaged somehow when attaching the pump head or possibly dirt got into the valve. Unscrew the valve core and inspect it for dirt, a bent pin or damaged seal. In any case there's no need to throw away the tube as you can get replacement Schrader valve cores for cheap at most auto parts stores if a new one is needed.

Thanks for the information, which I will certainly act on when I'm inflating my tyres.

I've broken a few presta valve cores over the years with my pump, but it stopped happening when I put a new pump head on. When they wear out you have to tighten it in a way that it wasn't meant to go and then it would break the core. You need to make sure the pump head is squarely on the stem so it doesn't put sideways pressure on the core.

...I just returned a batch of Continental tubes (which are pretty high quality products), because for some reason this batch all had defective Presta valves.
I had installed four of them already, so those I kept, but just replaced the valve mechanism. You can buy those separately, for both Schrader and Presta valves.

The usually come in quantities of a dozen or twenty. Or you can save them from other tubes that get spoiled for some other reason.

The Schrader valve core replacements they sell at auto parts stores are usually pretty good quality, because people use them in car tires.

ger yourself a valve core extraction tool or just a valve cap with extractor and start removing the cores from tires that get punctures and aren't repairable. Then you will have a ready supply of cores you can replace defective ones with.

Mystery slow leak from a car tire a couple of years ago. Realized I had a tire valve tool which I had never used, so I gave it a try. Was able to tighten the core enough to fix the leak. Have not had similar bike valve problem. Yet.

When I needed to loosen one while out on a ride I found that a spoke spanner fitted. Which I was very pleased about as, otherwise, I was looking at deliberately puncturing the tube to get it to deflate.

I've tried a few cheap tubes over the years. One that seems to be readily available over here is a brand called Vavert, which I've not been impressed with. I also had a mixed experience with Cannondale tubes, surprisingly. The best cheap ones I've found are Impac, which are made by Schwalbe.

When you replace the valve core, be sure to wet it first.
Saliva, though gross, works well....

Read. The. Post, people! This is not about a failed Schrader core. And it’s certainly not about two failed Schrader cores. The important bit you seem to be missing is “…after a few seconds it lost all its air through the valve.” Simeon Hope only thinks the air is coming out of the valve. Air escaping from a stem cut can often feel like it is coming out of the valve but it is coming out of the rim at the valve hole. It is not due to a poor quality tube nor to a loose valve.

Blaming the core is the reddest of red herrings.

Simeon Hope: inspect your tube. Pull the stem to one side and you’ll see a cut in the rubber around the stem. Alternatively, pump up the tube outside of the tire and feel for where the air is coming from. It will be immediately apparent.

As to the “why”, there are some different causes as I detailed in post 9 above.

+1

Are you using a frame pump that fits on the valve at a right angle? Those pumps can work really well and never damage the valve or inner tube at the valve base, but - proper technique is necessary. I use the Zephal pumps (the old HP and current HPX). I hold the pump just behind the head with my index finger wrapped around the pump head and my little and ring fingers around the spokes. This means that all my pumping force goes into my hand and the wheel. The rim rocks some but it is plenty strong, The valve just does what the rim does. No stress on it at all.

I rode sewups for 15 years without a floor pump. Every inflation was with those hand pumps. I killed maybe 2 valves being careless. (And none on my good race tires that cost like gold.)

I would carefully inspect the rim hole and smooth that all out if needed. Burrs are not unheard of. I make a "gasket" like a small donut out of Post Office tyvek envelopes (free at the PO). Put it over the stem then through the hole. I don't know if it helps, but I never have problems. Grape jelly will also repel elephants.

About that rim hole. I frequently find when trying to insert a schrader valve through the rim hole that the hole is slightly too small . I frequently will use a drill to slightly enlarge the hole and a chamfer bit (or larger drill bit) to relieve the edges so they are not sharp. Is there any reason I should not do this?

As long as you don’t raise another burr and you keep the enlargement of the hole reasonable, there shouldn’t be a problem.