Well that’s the question…
I have a vintage 1960s Hohner 26 (the grey and white one with metal casing). It sounds great and the keys are fine. Unfortunately, it is not as airtight as it should be, and when you blow through it without pressing the keys you can hear a hissing/whistling sound (not very loud, but irritating)…clear proof of air leakage. I posted a video of the instrument as a reply to my own ‘Roland Alphonso’ video on this forum, in case you want to hear and see the instrument.
At the risk of being obvious… disassemble and find the part that isn’t seated/is leaking. You’re probably dealing with an old cracked seal – either on the wind chest or one or more of the valves.
If no matter how hard you blow you don’t hear a whisper of a note, its probably the seal around the chest. I haven’t had that particular model in pieces yet, but my experience with other models indicates its likely a two-part plastic chest with a rubber gasket around the outside edge. Replacing this gasket would be my first attempt. Making sure that the plastic internals aren’t actually cracked while I’m in there. If they’re cracked, you could use some silicon caulk to seal it I suspect.
If it still leaked, I’d have to see about the key valve seals.