I have a really old (1906) pneumatic instrument that has about 65 small
bellows covered with rubberized cloth.  The cloth was deteriorated
to the point where many folds and even some flat areas were totally
without rubber -- you could actually see the weave of the cloth.

Since the pneumatics were firmly glued to the mounting boards I really
didn't want to knock it all apart if it wasn't necessary, so I thinned
down some Pliobond with acetone and applied it to the cloth.  I think
I used two rather thin coats.

I actually measured the leakage before treatment which was as much as
100 cubic inches of air a minute under 5" of vacuum.  After treatment,
it was down to 2 or 3 cubic inches.  That much can pass through the
wood and glue.

It has been well over 10 years and it's still tight.  Frankly, I was
amazed at how well the treatment worked.  The pneumatics started out
actually crispy and hard, and after treatment they were almost like
new cloth.

I had a long talk with the company that makes Pliobond.  One point they
made was that you should be certain to stir it up well because some
important additives tend to settle to the bottom of the can.  On other
occasions I've also used it successfully on large pneumatics.

Buy a small can of it.  It's easier to stir up and handle than the
little bottle with the brush inside.

Regards
Craig Smith