Hiya,
the reference for this is
http://accordionrevival.com/ACCORDION_REPAIR1.php#Air_Leaks
To find out if your leak is internal or external, measure the amount of time it takes the bellows to close under its own weight from the full open position with all valves closed. Rest your accordion on its feet on a sturdy table. Unsnap the bellows straps, grip one end of the treble keyboard in each hand, press a few keys on each end, and lift the treble section to expand the bellows. Dont mind the discordant sound as air rushes into the bellows through the random combination of open valves. Keep lifting until the bellows is all the way open and the bass section lifts off the table. Shake it a bit to make sure the bellows is fully extended, then release all the keys so all valves are closed (but don’t drop the accordion). Note the time on a clock with a sweep second hand and let the bellows close by itself under the full weight of the treble section. Dont let go, but dont hold any weight, either. Use your hands to keep the bellows going down straight, but not to hold it open or force it closed. If the bellows takes less than 35 seconds to close, it has some external air leaks, as nearly all accordions do. If it takes less than 20 seconds to close, then you probably have an external leak large enough to easily locate and repair. If it takes between 20 and 35 seconds to collapse, the leaks will be much harder to find
Its fairly easy to do once youve done it a couple of times. Fixing air leaks is more tricky but can be done yourself.
Ive got a Hohner Lucia that scored 25 seconds, its now around 70 seconds.
https://www.accordionists.info/showthread.php?tid=5666