Working on a high end Olds with issues in the tune slide/crook area, leaking air after re-soldering the two inner pipes onto the crook. Blow in one end, hold finger over other end PSSSSST leaking air..
Replace water key cork.... Still leaking badly..... (mutter bad words)
Attach 1 foot hose to one end, cork on other end, submerge in water..... Blow
Ah Ha! air leaking around little pad that has raised hole for water key pad..... Look close, its up and un-attached on one end. Remove and examine, crud city, been leaking for many years which may be why it was put in closet and not played much.Clean to bare metal, re-solder, Ta Da ! Tight like a tiger.......
Moral: Never give up on a stuffy horn.