The bellows supplies the air pressure that makes every reed sound, so even a minor leak makes the whole instrument work harder and can affect tuning and dynamics.
How to test
- With all register switches and any air-release button closed/off, gently compress or expand the bellows without pressing any keys - a well-sealed accordion should offer firm, even resistance with no hissing sound.
- Listen and feel closely along the folds of the bellows, the corners, and where the bellows meets the body on both ends, since leaks concentrate at seams and fold creases rather than the flat panels.
- A leak severe enough to hear as an audible hiss is significant; smaller leaks may only show up as the bellows needing more frequent direction changes than it should during normal play.
What it means for a purchase
Minor leaks at fold creases are a known, repairable issue (typically resealed by a technician), and are common enough on older instruments that finding one isn't automatically a dealbreaker - but it is a real repair cost to factor into an offer, and a good negotiating point if the seller hasn't mentioned it.
