Broodax

I am a very modest bike mechanic. I can change or patch a tube, set derailleur limiters, and I think I tried to true a wheel without ruining it once.

My wife's bike had a flat after a ride this week. The whole ride we didn't notice, but the next day it was entirely flat. I pumped it up and could hear and feel air escaping next to the valve stem (not out of the stem, it continued even when the stem was covered/plugged). I replaced the tube, no big deal. I assumed that the tube had been compromised around the valve stem hole in the rim or pinched, etc.

I inflated the tube after I had swapped it out, thinking I could patch it to be a spare again... and it just holds air now. It has no leaks. What the...

zandoval 

Don't trust it... Its a fairy tube just waiting for you to mount it and then fail...

Gresp15C

Maybe there was some debris in the valve. I'd inflate it, and hold it under water, looking for bubbles. No sense in throwing away a good tube.

Mad Honk 

I'd suspect the tube leaks around the stem under pressure. I ran through a batch of Sunlite tubes that were hit or miss. the failure was at the stem juncture with the tube at about 100psi. Installed them and then waited for five minutes and the leaks appeared. Without the higher pressure they seemed fine. I agree with the first opinion, don't trust it. Smiles, MH

CliffordK

When you pump it up to 2x size, then put it under water, also wiggle the valve stem. Look for cuts/cracks around the rubber or valve joint.

It is also possible that the hole is "near" the valve, but not at the valve. So, the air escaping through the valve hole, but only because that is the easiest path out of the tire.

Rick

Over many years I have had more flats from internal problems with the tubs than punctures from the outside.

shelbyfv

Agree. The tube has failed where the stem attaches. Inflate the tube, wiggle the stem to verify. I've been fooled by this more than once.

Pop N Wood

Getting a puncture from a spoke nipple/bad rim strip will cause that to happen. Air leaks into the tire cavity and the only place for it to go is out the stem hole.

When you change a tube pay attention to how the tube and tire come off the rim. when you find the hole in the tube, go back and check both the tire and rim for possible issues. A small piece of wire from a car tire could be stuck in the tire, which means it will puncture your new tube as well.

canklecat

Check the hole in the rim for the valve -- there might be a burr that nicked the tube at the base of the valve. I've had to carefully file and burnish valve holes on a few rims.

Spoke holes are often burred too on lower priced rims, but good rim tape will cover that up. But some rim tape/strips don't adequately protect tubes around the valve base.

sweeks

This reminds me of the first new bike I bought for myself (in 1975). It was a Motobecane Mirage, with steel rims. Inside of the first two weeks I had it, I had at least five flats... none of which was from a puncture. I tracked the flats to drain holes in the sides of the rims. I went back to the shop where I bought the bike and was told "Wheels aren't covered under the warranty"!
I had to chamfer the holes myself, and never went back to that shop. I still have the bike!

New front rim, handlebars, cables, brake levers, grips and all bearings serviced... otherwise pretty much original

curbtender

What kind of valve are you using? If you don't screw the presta down all the way it can seep air. I had a friend give me some tubes he thought failed and they were presta with replaceable valves that had come loose when he was taking the caps off.

Broodax

It's just a shrader valve. The others are likely right, I'll have to get it under more pressure and wiggle it more forcefully. I'm certainly not going to trust it without finding the leak and patching it first.

canklecat

Hey, that was my first serious bike too, a 1976 Motobecane Mirage in that nifty silver-blue paint. Steel rims too, which made braking fun in rain. I don't recall any problems with burrs but that may have been luck. Wish I'd kept that bike. I sold all my bike stuff about 20 years ago after a car wreck busted up my back and neck. Figured I'd never ride again. Took awhile but I recovered enough by 2015 to try again.

Gresp15C

Put some water right into the valve and see if the air bubbles out there. I've had Schrader valves leak, and then be fine after re-inflating. Also, the valve core is threaded in, and could be loose.

kctrading

I always use a spray bottle and some dish soap to find my leaks

sweeks

Congrats on overcoming your injuries! 20 years ago is about when I had some bad news from my annual physical and decided to take action. I used the Motobecane, which had sat mostly unused for years, to get back in shape. I lost about 30 pounds, then bought a mountain bike, then got into bike commuting... the rest is history! I just passed the 19-year mark commuting year-'round by folding bike combined with heavy rail.

drlogik 

Sounds like a faulty valve stem. I wouldn't use the tube unless it's a Schraeder valve, in which case, switch our the valve core and see if that makes a difference under pressure.

bikebikebike

Stem leaks are not uncommon and may only be conspicuous under pressure.
I see them more on older tubes, but certainly anything can fail anytime.
cracks in Schraeders, cuts, and seals to metal in both Schraeders and Prestas.
Can be spontaneous.
My last one was while I was sitting drinking coffee next to the bike.
Makes riding with a spare tube or phone more reasonable, usually only happens when I left them behind.

vane171

Besides wiggling the valve, also kneed the rubber around the valve with your fingers, squeeze it to bend the rubber around valve into fold or stretch it a bit. You can also try pulling on the tube with hands gripping the tube to the left and right of the valve.
But wiggling the valve should normally do it.

Sometimes it might be better, like in this case when you can't inflate to high pressure is instead of dipping the tube under water, make a soapy solution with a dish soap in a bit of water and brush it on the inflated tube and wait. Or wait a bit if testing underwater since in slow leak, it can take a bit before a bubble starts to blow up.

Don't expect to see a steady stream of bubbles in a second or two pause before going to the next tube section. I was surprised at times how devious underwater tube testing can be. If the bubble starts to develop underneath the tube where you don't see it, it might be inflating the bubble too slow for it to detach and float up, when you would see it. But usually you pass onto the next tube section and miss that bubble.

canklecat

Ditto, loose threaded valve cores. I never experienced that until around 2018, then it happened twice in the same year, once with a Schrader tube, once with Presta. My mini-pumps have valve core wrenches built into the end caps so I can snug 'em up as needed.

Cooperator

My suspicion is always a minor cut or tear at the base of the valve stem resulting from tire rotation on the rim. Especially common with older tires/rims. Leak not noticeable until high pressure or valve twisted to one side.