Over the last 25 years, I've used pneumatic cloth but mostly pouch
leather for Duo-Art accordion pneumatics.  For the last 20 years I have
stuck with the leather.

Yes, it can stretch and become porous, but there are ways around those
problems.  You must use only the best German tan leather of a medium
thickness.  After applying the leather to the accordion pneumatics,
I run a 3/8-inch strip of Nylon pneumatic cloth down each side, running
it behind where the adjustment blocks mount, and positioned as to allow
one screw from each block to go through the Nylon strip.

Then I rub the mixture of Dow Corning 111 silicone grease, mixed with a
solvent, into the exposed surfaces.  Two light applications seem to
work best.  Let the solvent evaporate and dust the accordion with talc.

The leather provides easy and smooth movement, the Nylon strips prevent
leather stretch, and the Dow Corning silicone mixture prevents leakage
and leather deterioration.

The accordion pneumatics that I covered with pneumatic cloth have
required recovering, as well as the ones done in leather without the
sealing with the silicone sealant.  They also had the stretch problem.
The ones done in leather with the nylon strip and the sealant seem to
be holding up fine.

I agree with Jim Heyworth on the covering used in the expression unit
that he purchased from me.  It was pneumatic cloth.  But I wonder --
Could a factory engineer trying to cut cost have talked them into using
pneumatic cloth?

Al Pebworth
Pebworth's Player Piano Center
Home of "Yesterday's Music"
http://www.pebworths.com