I just gave up on an MTB tube. I patched with (w/glue) where I heard the leak...but it slowly went down again. Pumped it up (outside the tire) to get it pretty big and could never detect a leak...even stuck it underwater in my pool (60-degrees is REALLY cold) and couldn't see a leak...but a day later the air was gone. Not worth further investigation...maybe it was the valve. dunno.