I have an older, but very lightly used Thermarest mattress. No matter how tight the valve is closed, the mattress leaks flat. Never been damaged that I am aware of, and can't find a leak visually or by sound. I'm guessing the valve is bad, but who knows?
Anyone know if they can be diagnosed and repaired by the manufacturer? I've even tried submerging it in a lake and couldn't really find a leak.