A silly boy once restored a BSA Bantam and when he came to put on his nice new tyres he thought "those old inner tubes look OK, I'll put them back in". And every week he came to ride his motorcycle, the tyres were flat and he had to pump them up.
The old inner tubes were made from some nice butyl rubber which like most rubber was natively good at passing small molecules through its elastomeric matrix. However as this rubber was very, very old and had become, over the years, very wise, it had learned how to pass the small gas molecules through its matrix very well and was now highly accomplished at the art.
But, the time comes for us all and after a while the boy realised what he had done and decided it was time to retire the old rubber and he took the shiny new tyres off again, and replaced the 60 year old rubber, with nice new tubes.
The new tubes still haven't learned how to leak, but the boy has learned not to scrimp on inner tubes!
But for now, dear Amelia sits in her shed with her old tyres and tubes which leak. Since I have no need to remove the tyres just yet, as I want to sort out lots of other things first, I have taken this handy can of tyre weld and re-inflated Amelia's tyres.
They are now full of fluffy goo. I'm told this is not good for tyres you want to keep, since once they are contaminated by the goo they cannot be repaired.
And so now dear Amelia can roll about on hard tyres and sit outside her shed in the sun. Here she is, ready for me to sort out her side stand, and her primary chaincase, and the gearbox.
Isn't she lovely.