Reply To: Leakage
The Piano 36 is a bit trickier to work on than most melodicas. Yes, you are right. It would have been better to live with a bit of leakage than to take that particular instrument apart. A lesson for everyone. But I probably would have done the same thing. Some tips are provided below.
Vintage Hohners leak. I readily accept 5+ percent leakage with moderate-to-high blowing pressure because when you are playing the melodica, the valves are open, the pressure is lower, and there are no leaks. (Obviously this is not true with significant leaks at low low-pressure.) Many of my vintage Hohners will leak water after playing a long time. Again, less than perfection, but nearly the norm for vintage Hohners.
Now to the solution. I took the back cover off one of my Piano 36s that I personally bought new, and noticed that in addition to the gasket, a white caulk/sealer was used at the four corners and the ends of the peak of the “roof” of the back cover. You have probably disturbed those sealing points. You may have to completely remove the back cover, inspect all sealing surfaces, remove the old caulk/sealer, put down a bead of new caulk/sealer, and reassembly using the techniques described above.