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Try a little bit heavier rotor oil to see if that helps.

Try blocking off the tuning slide and then pushing 1-2-3-4 and see if the IS any air escaping when blowing into the mouthpiece!

If the weather is not too cold where you are, try sticking the end of a garden hose into an old mouthpiece, inserted into your leadpipe and turn on the water. Watch for water squirting out where it shouldn't. You also may flush out some paper or some dried green crud which sometimes flakes off the inside of the tubing and flaps around in the breeze. (Also try the suggestions above.)

have you asked a friend to help isolate the leak?

No, its not normal for a horn of any age. The venting of rotary valves (if its done) DEPENDS on no air leaking into the valve slides from the open valve and vice-versa.