First thing is to get the horn reasonably leak free. You need to put a light down it, identify each leak and why it's leaking. Sometimes it's that a pad that used to seal has become distorted or deteriorated. Sometimes it's that a key has gotten slightly bent. Sometimes the pad would seal just fine, but the key interactions have gotten out of adjustment and it doesn't close properly (this in itself has multiple root causes - piece of cork fell off, piece of cork has crushed down, something bent, etc.) Sometimes it's a looseness in the mechanism that can be easily fixed by tightening a screw that's worked loose. Sometimes it's a looseness in the mechanism that needs tightening up due to wear - again, several different ways to reverse the effects of long term wear. Sometimes a tone hole has gotten distorted, maybe by a nearby dent, maybe a dent on the tone hole chimney itself. Sometimes a pad was badly installed and has shifted and distorted till it no longer seals. You also mentioned apparent leaks at joints in the tube itself.
Until the horn is reasonably leak free, and your initial crude inspection showed that yours leaks like a sieve, everything else is just wasted effort.
someday I will use them
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