A further note for those who haven’t opened one of these up. In the first photo you see three small round holes inside the reed cover. It is through these holes that air passes through to the bladder. So there could be leakage from the bladder or from the reed chamber itself. It would be interesting to try blocking the holes to help isolate leakage, but also to see how the melodica plays without the bladder. I tried blocking them by pressing down the bladder on those holes but couldn’t do it effectively.