You could try pumping the tube up outside the wheel and leaving it and seeing if it goes down without going anywhere near the wheel and/or tyre. Then if that holds the air well, try it just on the wheel without the tyre. etc, etc.
@ilikemybike started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
You could try pumping the tube up outside the wheel and leaving it and seeing if it goes down without going anywhere near the wheel and/or tyre. Then if that holds the air well, try it just on the wheel without the tyre. etc, etc.