I do not own one of those models so I do not have any firsthand knowledge about the design of the melodica or the water valve. However, here is some general advice.
If you feel adventurous and have some mechanical aptitude, you could remove the back cover and inspect and adjust the water valve flap. It may not be pressing hard enough against the drain hole. The foam could be deteriorating and causing a leak, but I have yet to see any deterioration of foam or adhesive in a Hohner melodica no matter what the age.
Just tightening the screws on the back cover might eliminate some leakage if the screws are loose.
Secondly, you could send your melodica to the Hohner Shop and have an expert service it. You download the “reconditioning Service” form at:
I will be sending in a few melodicas soon to the Hohner Shop and will report on my experience. They have actually sealed up a couple of my melodicas under warranty and did good work.
Finally, I think just a bit of leakage (5 percent) from a 50 year old instrument is about the norm. Some are fully tight, some leak a bit. Several of my old Hohners leak water just a bit from the gaskets or water valves after extensive playing. I notice this because I hold my melodicas at the end opposite the mouthpiece, and my left hand can feel the water sometimes. That is why I often hold them with a decorative and absorbent cloth in my left hand.
A 50-year-old Hohner is like a 1969 Chevy with 175,000 miles on it. It is probably going to leak some fluids.