Originally Posted by
skycyclepilot
I noticed that something was off during a 25 mile ride. I stopped, and the rear tire was low. I could pump it up and get about five miles out of it before it needed air again. Most flats I've had were instantaneous, or nearly so, so I call this one a slow leak. I'm aware that all tired suffer from air osmosis. That's why they have to be pumped up before nearly every ride.
Fill a sink with water. Take the tube off the wheel and pump some air into it so that it is slightly pressurized. Submerge the tube in the water and look for air bubbles. You may also be able to hear the leak once it is off the wheel. Without the tire there, the leak will likely be faster than on the bike.