So I have a total of five of the pneumatics off the piano. Yes, the looked good. Yes, they leaked when blown into and sucked with one's mouth. Yes, all were recovered. Here are some pictures.
Here you can see number 13 and 14 together. 14 was re-covered and passes both a blowing and sucking test. #13 kind of passes it but as you can see the flexible sealant makes the overall pneumatic shorter and it doesn't move as well. So recover the thing! It's not totally impossible.

A view up in the piano. The glue does not look too great there; I'll remove it to make a clean surface then re-install with a bead of fresh glue. Hide glue of course. It's going to be a bit complex as I will be doing it upside down, will hold in place for 5 minutes then use tape to keep it up till the glue solidifies.

This is.... very interesting. It looks as if dad used a paper gasket or something between the bellows and the board. Not sure what that is or why, but maybe it was the style of repairs in the 1970's? Don't know.
The one on the right is even more interesting. It looks like the glue was overheated and applied burned or something. I do not know. What I do know is that all of this glue is coming off....