Whenever this happens, the first place I start my looking into the problem is the trill keys on the upper joint. Particularly in the case of the B and C keys, they are exposed to damage much more than anything else up there, as a slight blow can move them off of the normal alignment, even though they are set in that bracket that most horns have. Leblanc trill keys are also "different" in that they are set up as "jump keys" where one key vaults over the key cup of the one below. I have always found Leblanc trill keys to be more prone to this leakage issue, although all of my experience with them dates back to a six month period where I used a Leblanc bass. In any event, take the cheap approach of checking the sealing of the keys before you start in with more drastic measures.