Quote Originally Posted by Patrick Huang View Post

There is another minor problem but I don't know its cause. Sometimes the key is not rebounce and it sounds still when doing nothing but stepping the pedal, but it could be solved by repetitively pressing the corresponding key. I have recorded a video about that:
The key staying depressed is most likely the small dowel rod (incorrectly called pitmans) that binds in its hole. These rods were originally coated with graphite powder or similar to make them slide easily. This has almost certainly worn off by now and perhaps the hole is roughened some as well. Perhaps this will kind of remedy itself by some vigorous playing.... (tongue in cheek...) In any event this is probably possible to fix with less difficulty than the second possible cause: A weakened pallet spring. To remedy this the wind chest and reed pan will need to be separated involving more careful work. On an old one like this that possibility is rather more likely and redoing all the insides might be necessary even at a later stage.

If the exhauster valve is obviously not working properly try and fix that first, also the side-rinding strap. While testing and playing the sticky key may come back to normal working.

Have you checked the bellows for leaks?

Good luck!