Haven't made much use of my four and a half bikes over winter, but pumped up some tyres a couple of weeks ago. This week I found the front tyre of the Moulton DP was flat, so took the tube out for examination. No patches on it, bubble test in water gave no results, so put it back assuming I hadn't tightened the presta after pumping it.
Flat again today, so off again and this time pumped it as hard as possible for another bubble test; seems you can't get over 20psi because the bare tube just expands above that.
Another bubble test revealed a very slow leak from the valve, so have tightened the valve stem a bit more and, hoping for the best, put it back again. Hope it's fixed, but can't understand how, after a few years' use, it slackened off.