My son has 4" tires that run at 20 psi. We recently had a flat and replaced with a presta valve & tube. The problem is about 15 minutes after pumping up and locking the nut down, the valve will release about 1/3 pressure.
Nut stays tight and it's a sudden release.

Was it presta before? I can't think of any bike tire that only has 20 PSI in it. 30 maybe more are 45 PSI +. If it is a big hole rim for shrader valved tires it has probably caused a hole in the new tube. Roger

4"

A fat tire bike will run at low pressures.

Perhaps hold a glass of water up to the valve and verify that it is actually the valve that is leaking (or spit on it). Many presta valve cores can be removed and replaced.

It was a new tube set from REI it came with the presta valve on it. The rear tube and front tube were both done at the same time. It's only the front one with the issue. And yes it's definitely the valve I've had my hands on it while it's released.

Maybe the valve has torn a little from the tube. Check the base of the valve where it connects with the tube. It may hold air a while but if the valve moves a little, it will release air at the base of the valve. A valve that has torn from the tube cannot be repaired, needs to be replaced.

Thanks for all the help. I've patched and slimed the old tubes and will be playing with the presta one outside of the tire but so far can't get it to behave like it did.

Here's probably a dumb question. I do not have a dedicated presta valve inflator, only have a couple bikes with presta's. I use those screw on adapters to use a schrader type inflator chuck. Is it acceptable to leave the adapter on and put a schrader valve cap on it or must you take it off. I've been taking them off. I bought a whole card of these adapters and was wondering if there is an "official" stance on this?

You might fill the tube and then hold the valve area in a sink of water to see whether you spot any air leaking. Wiggle the valve around to help expose any flaws.

You mentioned tightening the little nut, which makes me wonder if in doing so you tore the valve somehow.

Possibly you just have a defective valve.

Have you gone through the 15-minutes-then-lose-pressure cycle more than once?

Not a dumb question in my book, will watch for an answer. In the meantime, I wasn't aware of such converters and given my disdain for Prestas will seek some out. Thanks.

I have always left them on, when I used them.

Most of my bikes are presta but I keep a schraeder adapter in my patch kit. It's fine to leave it on the valve, but I'd be a little worried about a jerk stealing it.

OK, I'll leave them on. I was thinking they might cause a leak. Good to not have to go looking for parts just to air up the tires. Good compressor type presta inflators are not cheap.

Any opinions on drilling out presta type rims like the Sun m13's to use schrader tubes. I did this on another bike because I internet bought tubes and rims at the same time. SURPRISE, presta rims schrader tubes.

Spray the thing with soapy water. It may be releasing through the valve, or (more likely I think) at the base of the valve where it joins the innertube. Some dish soap in a small glass of water, applied liberally. You should see bubbles forming.

Leaving it on is a convenient way to make sure it's always around. Just don't forget to take it off and tighten the Presta locknut again after filling. Loose Presta locknuts can allow the valve to jostle open momentarily and lose air.

Yes, leaving the converter on is no big deal (unless you're a weight weenie worried about the rotating gram that unbalances your wheel), but after inflating I would take it off, close the Presta valve, and put the converter back on, otherwise the valve could be very leaky

Offhand, do you know the diff in the size (diameter) between Schrader and Presta valves?

https://en.wikipedia.org/wiki/Schrad...er_valve_types
I've seen adapters before for going the other way, basically a little ring with a lip you drop into the schraeder valve hole, so the presta valve doesn't move around and/or get cut into by the edges of the rim

I'm new to presta valve myself, but I found some good info on that subject at Sheldonbrown.com

Yes we've done it maybe 6 times. I checked and double checked for holes or pinches. After taking it out and putting the old tubes in I held it under water and no leaks. However, it hasn't done the random pressure release now that it is outside of the tire. I'm thinking about putting a higher pressure in it and seeing if it does it but the tube expands so much more now it's not inside anything. Will it blow out before registering a pressure on the pump?

The tube won't explode by pumping it up outside of the tire so, yes, it sounds like you need to pump it up a lot more than you have been to expose the leak.

Please confirm that you have wiggled the valve around after pumping up the tube. A small leak at the base of the valve does not reveal itself even underwater unless you move the valve around.

It may have just needed the seal to set into the valve. 20 psi is pretty low. I've used a drop of oil before to help it out.

an Inner Tube that is Not constrained by a tire will stretch so that the volume of air inside it exerts equal pressure as the surrounding atmosphere. For an apparent gauge reading of 0. Until it ruptures.

You're not getting a repeat of the first pressure drop because unless you actually have the tube inside a tire, you Aren't pressurizing it.

My old Serfas Airbones pump chuck used to destroy Presta valve cores until I switched to Silca presta valve cores. Saved me from buying a new tire pump.

Thanks, will have a look.

Do you mean you switched to a Silca chuck?

I've got the Nashbar L'Orange V2 pump, and I like the concept but man is it hard to pull the chuck off! I've gotten into the habit of spitting into the chuck every time I use it so there's a little lubrication in there.

I once ripped a schraeder valve right out of a tire!