deflating tube with no air loss

[Redlight]

Fitted a new tube in the front tyre of my wife's Brompton after she punctured. Tyre checked, as always, but it looked like an impact puncture as she'd hit a pothole in the rain.

A week later, she hadn't ridden the bike but the tyre was a bit soft. No worries, pumped it up and all seemed fine.  She came to ride it for the first time a couple of weeks after that and the tyre was soft again. By soft, I mean malleable but not flat.

So, I took the tube out, checked the tyre again. Nothing. Pumped it up and popped it in a bowl of water to find the leak. Not a bubble in sight. Deflated it and tried pumping it up while in the water. Still no bubbles. Not even around the valve.

It's been hanging up for a couple of days now, fully inflated. No noticeable softening. (There's another tube in the tyre.)

Am I missing something really obvious?

[Lightning Phil]

Is the valve core nice and snug?

[Ian H]

Air being lost that slowly can be very difficult to spot.  Most likely a slightly leaky valve.  If you have the patience to hold it under water for some time you might see a bubble gradually forming.

[Redlight]

Air being lost that slowly can be very difficult to spot.  Most likely a slightly leaky valve.  If you have the patience to hold it under water for some time you might see a bubble gradually forming.

I'm not sure I have quite that much patience, but I'll give it another try for, maybe, five minutes. 

[Kim]

IME, dodgy Presta valves tend to leak intermittently, depending on how tightly you've done the wossname up.

ETA: Although re-reading the OP there's a strong possibility that this is a Schrader valve.  I suppose a loose core or minor grit contamination might have a similar effect.


Meanwhile, my own Brompton's recent mystery deflation turned out to be chemical contamination (I assume chain lube) seeping through a spoke hole into the rim tape and slowly dissolving a patch of the tube, which explains why it seemed sensitive to ambient temperature, and got progressively worse over time.  I've cleaned it with alcohol and replaced the rim tape, so hopefully that will solve it.

[FifeingEejit]

Interesting, I've never had a tube that doesn't lose some pressure over time, some are faster than others