Concertinas have two reeds controlled by each key (button); one that sounds when you are pushing the bellows closed and one that sounds when you are pulling them open. Each reed has a one-way valve associated with it so that air doesn't flow 'backwards' through the reed. The valves are simply flaps of thin leather covering slots in the reed board. If a valve is missing or not sealing properly, at best you waste air and at worst the reed can make some very weird sounds. There are 96 valves in a concertina like mine.
On my concertina, most of the valves were either perished and falling apart (I suspect those were the originals) or they had been clumsily replaced with much thicker leather that wasn't flexible enough (actually, I think it was the same leather that the thumb straps were made from!). ( Photos behind the cut...Collapse )
Time spent replacing the valves: about 8 hours over several evenings.
 The thing that opens and closes to control the flow of air when you press and release a key is called a pad, not a valve.
 On English concertinas the two reeds associated with each key produce the same note, so if you hold a button down and continuously push and pull the bellows you get a bagpipe-like drone (with a slight hiccup each time you change direction). Other types of concertina play different notes on push and pull.
 Carefully laying them out on a marked sheet of cardboard to make sure they all went back in their original positions.