I am away from home, Andre. Maybe I’ll take my 32A apart when I return to see if that gives me any ideas. But for now a couple of things.
Melodica-Me suggests blowing cigarette smoke through a melodica to more precisely locate leaks. This will not damage your lungs unless finding melodica leaks is your life’s work.
If a key pad is very slightly open, it will release some air when you blow without any keys depressed, but the note will not sound unless you blow it hard enough. But I agree that’s probably not your problem.
Sometimes caulking cord is useful in experimenting to find where a melodica is leaking. It can be applied in strips or in small dabs, shaped to the contours of where it is being applied, compressed, and then easily removed and cleaned off. It can also be left in place if it solves the problem, though it may have to be replaced when you take the melodica apart later. It is sold on Amazon and in hardware stores (not sure about in Brazil), has many other applications, and is inexpensive for enough to do melodica repairs for the rest of your life.