In regards to the never-ending quandary whether to seal leather pouches
or not, what was the reasoning for sealing them in the first place?
What I feel was in the minds of our predecessor builders and
restorers is that, by sealing the pouches, you're leveling the playing
field.  All untreated/unsealed pouches are porous to a certain extent.
Even if the pouches were punched from the same skin, there would be a
variation in porosity, ever so minuscule, making the reaction time vary
from pouch to pouch.  By sealing the pouches, the designers of the
pneumatic systems were evening out any variation and thereby creating a
uniform reaction time.

This way you could have a standard for setting things like bleed size,
pouch size and dishing, and valve travel, throughout the entire
mechanism.  This is especially necessary for reproducing piano
mechanisms.  Ampico mechanisms employ unit valves.  Their unit valves
were manufactured to a strict standard, so that you could replace a
defective valve with another from the factory without having to
regulate the replacement valve to match the response time of the other
valves.  Sealing the pouches generally insures that all the valves will
function as originally designed, from quick initial response time to
recovery and speed of repetition, whether it be a valve that was
supplied when the mechanism was first installed or a replacement
valves.  You must remember that a reproducing piano mechanism operates
under varying vacuum pressures, so that it was critical for the valves
to function evenly throughout the entire mechanism, while enduring
drastic changes in vacuum levels.

So, what's the problem with sealing leather pouches?  Obviously it lies
in the type and amount of sealant used.  Unit valves of various
manufacturers were generally not designed to be rebuilt.  Instead,
replacements were to be ordered from the factory by an authorized
technician and installed, while the defective valve unit was to be
discarded.  Aeolian (not to be confused with Aeolian-American), appears
to have used some of the finest-quality materials in their instruments.
Whether it be pneumatic cloth, rubber tubing, or leather, most have
stood the test of time.  I had once opened a portion of an Orchestrelle
and found that the leather covering the pneumatics was still soft and
supple as the day they were sealed inside at the factory.  Whether the
leather was supplied by outside manufacturers that way, or treated by
an in-house secret formula, we may never know.

D.F. Rowe
Bronx, NY