RoadWearier

I have a front tire that keeps losing air. I recently replaced the tube. Pumped it up and two days later it was completely flat. I thought maybe when I had replaced the tube and was doing other things with the bike, maybe I had forgotten to pump up the tire. So I did. Two days later still flat. So I took the tube out inspected visually and in a bucket of water. No leaks, no bubbles. Ran my hand around rim and tire...nothing. valve hole is smooth and now has tape inside it. Anyone have any ideas? Valve was submerged as well with no bubbles. I have replaced with another tube and am going for a ride now. I have also blown up the tube and have it outside the tire to observe. It was a brand new tube I actually bought at the LBS so if it is defective somehow I may ask for a replacement.

Crankycrank

Pump up the tire to max. safe pressure and submerge partial sections of the tire at a time in a tub or ???. I've found some near impossible leaks to spot doing this. Also, when you find the leak, check the area all around rim and tire for tiny bits of puncture producing crap. Also try wiggling the valve when underwater as sometimes a tear near the valve will not leak until moved. Make sure the valve is tight in the stem also.

RoadWearier

Well like I said in the OP. I tried that. Submerged the whole tire piece by piece in a bucket of water and saw no bubbles. And again I checked tube, tire and rim. Nothing there.

bboy314

One possibility is a loose valve core, so check that. But sometimes a tube just has such a small puncture that the bucket test doesn’t work.

JohnDThompson 

Yes, I just had that happen last week.

Pull out the valve core, squirt some sealant into the tube, reinstall the core, install the tube and tire on the wheel, pump it up and ride it around a while. The sealant should be able to plug a tiny leak.

downtube42

Remove the tube and inflate it fairly large. Put it in the trunk of your car for a few days. If it's still inflated, you have gremlins. Otherwise, is a slow leak and must be found with extreme amounts of patience

Grab a number of beverages of your choice. Fill a bathtub with water. Turn on all the bathroom lights, which should be bright. If not, acquire lighting but don't electrocute yourself. Pump the tube up large again. Frighteningly large. Whether you get in the bathtub yourself or not is a personal choice. Now painstakingly, inch by inch, rotate that tube through the water. Ensure everything is submerged including the stem.

If nothing, throw away the tube and tire and start over.

Or skip all that, throw away the tube and tire and start over.

Edit: if you do find a hole, find what made the hole. Otherwise you're here again.

Andrew R Stewart 

Yes. At some point one needs to just move on with life and prepare once again for that next flat tire.

I have fixed many flat tires when the cause wasn't found out including those that didn't even leak during the out of tire tests. So when talking with riders and customers I have used the term "high pressure leak" (don't remember where I first heard it used) to describe a leak that only shows when there's more than a few PSI in the tube. A tube outside of the containment of a tire/rim won't ever have much PSI as it will just expand. The high pressure leak will only show its self when in a tire/rim.

A situation I have seen too many times is a fine piece of wire that "wove" through/between the casing's threads and only "tickle" the tube. When a finger is rubbed around the tire's insides in one direction the wire lays flat and is undetected. Reversing the direction of your feeing for the cause might pull the wire's end up a tad and be more likely to be felt. Unfortunately the tires that have a woven breaker ply (can you say Kevlar Belted) don't stop these bits of wire as well as tires with a plastic "Tuffy Like" layer molded into the casing do.

Another manor of flat tires that I have dealt with are when the tube has become abraded from the tire's inner surface (which often have nearly no layer of "rubber" covering the fabric threads). Usually from an under inflated tire (can you say "chicken or the egg?") that compresses too much as you ride. The resulting squirm between the tube and casing inside will sand down the softer tube. When the tube is removed one will see what looks to be "rubber dust" (guess why...). No specific hole but enough reduction of the tube's wall thickness can be enough to have a slow leak down happen.

Which brings me to that question that I want to ask the customer, but won't as it's not nice to be combative when working retail/service, "What's the difference between a flat VS a slow leak?". But instead usually ask "when was the last time you pumped the tire up to the labeled pressure?". Andy (trying to avoid so many rants about fixing needless flats for others)

rm -rf

I had a very slow leak recently that took 2-3 days to go flat. Dunking the tube in water didn't find the leak. I wanted to locate it so I could check that portion of the tire for a wire or tiny sliver.
So I did a more careful water test -- this did work:

Fill a basin with water. I let it sit overnight, to reduce the air bubbles in the water. (Maybe this step is optional?)
Hold a section of moderately inflated tube underwater, then use a finger to wipe off any tiny air bubbles stuck to the tube. Go all the way around the tube circumference.

Look for a slowly recurring bubble! My leak took at least 10-15 seconds to start forming a very tiny bubble after I wiped off the previous bubble there.
wipe it off once more to verify.

Troul 

Fill up a tote/tub with room temp/warm water. With the tube & tire still in the wheel, pump it up to the max rating PSI as labeled. Put in a few good sized drops of dish soap in the water & submerge only about a 1/4 of the wheel into the soap laced water. If after a few minutes you don't notice some bubbles forming, rotate the wheel a 1/4 & continue to observe for bubble action. Keep on it until either you find the leak or you don't see any bubble activity pointing to the sus puncture.

Andrew R Stewart 

When I use a water dip to find a hole I will usually encircle the tube with my fingers and slide the hand/fingers along the tube's length as it is under the water. I find a couple/three go rounds will be enough to remove the surface bubbles (from the tube's surface) and sort of clean off the tube to better see the start of the bubble. When the valve is under water I'll poke the pin and see if any air continue to bubble up after the trapped air get's blown out.

For my personal flats I carry a spare tube or two and pieces of pack cloth for booting, leaving the patch kit as a last resort. Still i try very hard to find that cause even if it means taking an extra few minutes at the side of the road. The various tubes I have collected over a season will get a group patching party, often during some winter sporting event that's on the radio. This is when I take the time to find the hole in water at home. Andy

striker65

Holding the tube close to my face can sometimes find a leak.

staehpj1

I quit submerging for tests and started using a spray bottle of something like soapy water. In my case I just use Windex. It may take a few minutes, but even a tiny leak will make a little pile of tiny bubbles. You may need to wipe off bubbles to see if they reappear.

grumpus

"But it was fine last year when I pumped it up."

Kontact

Tighten the valve core.

FBinNY 

I've had mystery leaks like this that ONLY showed up underwater when the mounted and fully inflated tire was submerged. Most were the result of poor molding of the cone over the base of the valve. They could also be the result of a poorly molded tube that was very thin in places, or hyper-stretched when expanding down past the beads on narrow rims. (the damage from this is usually visible along the belly, especially near the valve)

FWIW seeing the same issue with multiple tubes generally rules out the tubes themselves, and points to something bigger picture like the second possibility I mentioned.

soyabean

A bucket of water is too shallow to gives very little water pressure to squeeze the air out of a leaky tube (think the Titan submersible implosion).

Fill enough air into the tube so it no longer bends or kinks. It needs enough psi so it wants to leak.

Now do it in a shop sink or bathtub with at least a foot of water in it.

You will see it 100%, that includes any that will escape thru a bad valve.

Reynolds 

Never had a puncture I couldn't locate. Last one was a 700x40 tire, pumped up to 60psi, 40 the next day, flat the 3rd day, Took out the tube and pumped it to double its size, about 15psi. Tested the tube in about 15cm of water, a very small bubble formed every 6-7 seconds. The leak was at the removable Presta valve base. Took the valve out and put a patch with a 6mm hole punched in the center. Reassembled, inflated, worked OK.
Now you can say that was a lot of work to save a tube, but I have the time and actually enjoy doing these repairs (this tube had about 15 patches on it already, but why toss it if it can be saved).

DOS

You may have a vey small bit of glass or wire in the tire that is causing small tube puncture when tire is inflated. I was having this problem and eventually discovered a small bit of wire penetrating my tire. Holes in tubes were imperceptible until tire was inflated to almost max pressure.

grumpus

Interesting physics, except atmospheric pressure is ~32 feet of water, so one foot is insignificant compared to the one atmosphere exerted on the water's surface and several psi in the tube. Perhaps it's just easier to see a stream of bubbles when they have to travel further.

Duragrouch

Beat me to it. Lips even more sensitive. One time, I had to stretch the tube section that was in front of my lips, do the next section, and eventually found the leak. And note, first the outside perimeter of the tube, and then if not found, the inside perimeter. Water bath is usually the gold standard, but the lip test is great for a flat in the field.

shelbyfv

Eyeball is sensitive as well. Remove sunglasses....

MilhouseJ

It's probably still fine. The flat part is only on the bottom, just turn it over.

Andrew R Stewart 

Cats use this but with their whiskers being the sensors. I can feel a leak on my beard often when I can't hear it. Andy

Duragrouch

OK now I'm picturing a bearded cat. That's not good, that's not good.

ARider2

I pump the tube so it is fairly rigid and then I hold it near my ear and face using both hands I stretch the tube in order to open up any pin holes, then I move the tube a few inches while I listen for air escaping while I stretch and pull the tube around from one end to the other. Often I hear or feel the air rushing past my fingers as I handle the tube.