I agree that the pouch pneumatics must be flexible and responsive.
I did a little experiment some time ago.  I found that the red nylon
pneumatic cloth available from Schaff was more flexible than the Schulz
cloth from Player Piano Co.  I have not used the red cloth for a total
restoration, though.

It is just as important that pneumatics remain flexible.  I have used
Schaff's cloth for at least 20 years with no changes found.  I have
used PPCo's cloth for recovering Duo Art accordion expression pneumatics.
I am finding many times that they have become very stiff and no longer
able to function.  Fortunately, it is easier to re-recover accordions
than to re-do pouch pneumatics.

I currently have a Schulz-type piano in the shop.  It was rebuilt
about 1980 using leather.  In a general over-view test, it pumped up
well and was responsive.  I see nothing wrong with using good quality
leather except for the thickness compared to the original cloth.

I will probably use Schaff's cloth on my next Schulz restoration.
I have great deal of it on hand am comfortable using it.  Leather will
be a good second choice, although I usually only have about one skin
available.  Silk, gray, Schulz cloth will not be a choice.

Jeff Davis - hunkered down in dark, gray Seattle