I have a lovely old Jeffries 30 button that I recently purchased. It had been completely rebuilt in the 1970s by Colin Dipper, and is a wonderful player. However, it has developed an air leak. Investigating, i found the culprit -- a hairline crack has opened up in the action board, extending about 3 inches from the air button hole down to a nearby note hole, along the grain. The crack is only wide enough to see daylight through, but it is enough to render the instrument sluggish and leaky.
The soundboard, built during restoration, appears to be spruce or some such, and is single wood (not a ply). In later days, Colin used plywood for these boards, for just this reason....stability and resistance to cracking; my 1991 Dipper's action board is made with ply.
No doubt the change in climate from the UK to here in Texas had something to do with this crack developing. And of course, there is the issue of dry air from inside heating during the winter.
I'm thinking of spreading some wood glue into the crack, on both sides, to seal it. Or possibly some rubber cement. Any suggestions on this from seasoned repairers?