Tyres self-deflating after being in storage I recently took an old Brompton bicycle out of storage after several years. The tyres were deflated (unsurprisingly) so I pumped up the inner tube to check for punctures. I couldn't find any obvious punctures and the tubes did not deflate over the course of the next couple of days.

However when I put the tubes back onto the wheel they deflated within a few hours. Is this because the the inner tube has weakened over time and the pressure of the tyre is forcing the air out? Or is there another reason?

Many thanks in advance for any advice/help.

Chris
If you pumped up the tubes outside the tyres, you probably didn't pump them up very hard (or they would have gone bang). There probably wasn't enough pressure for them to go down quickly so you didn't notice. On the bike at proper pressure they're going to go down much faster (on top of that you'll notice changes more easily). So you could easily hold 5 psi for two days but not hold 60 psi for a few hours. 

A slow puncture can be hard to find even when you know it's there. So it would be even harder to find if you're not certain. 

Anyway I'd just get new tubes. If I suspected a puncture from sheer age, I'd be wary of sudden failure (in a different spot).