I have recently purchased a new Falcon box. The seal is so good its like a vaccum cleaner or ziploc. The problem with this can be that if you are playing a really fast tune the buttons can sort of feel stuck if you know what I mean. If the box is perfectly sealed then the only place the air is traveling is through the reeds and buttons. I have recently learned to sometimes leave my air button slightly cracked at all times while playing fast melodies, this allows for "looser/free" buttons. I have played several older accordions with not so great seals and this is not needed.
I have always been told that many older players such as Iry Lejeune always held the air button down while playing. I love the way he played. Can't be so bad...