1976 PA-18 "150" fuel selector valve leaks. Drained fuel completely and installed new (tiny) O-Ring and reassembled. Flew the aircraft and did not note any leaks. But when the engine was shut down and the aircraft sat for approximately one hour I noticed a "wet" finger when checking the fuel selector valve again. Is this a special O-Ring or is there some kind of back pressure which is causing it to leak? The new O-Ring was obtained from an auto supply outlet. Is there a special O-Ring for this? I didn't note any special O-Rings in the Univair catalog.