This discussion has gone on entirely too long.  Contrary to what some
say, this is not a "black and white" issue.  The bottom line is some
articles appear to be talking about sealing old pouch leather while
others are talking about modern restorations in which the pouch is
sealed when it is replaced.

Would everybody please define what you are talking about?!  Let's not
even discuss patch-up jobs since fine performance is not going to be
possible in those cases.  In quality rebuilding, the object is to
achieve optimum performance that equals or exceeds the intended factory
fresh performance.

And secondly, would everybody please acknowledge that pouch porosity,
bleed size, operating pressure, and other factors are not simple issues
that can be summarized with a simple "one size fits all" answer.

In general, if the instrument plays at low pressure, pouch porosity
becomes more critical.  Another generalization is, if the pouch
operates a percussive instrument action, the pouch is more critical
than if the action is for an organ pipe.  In the case of a percussive
strike, the note might not sound at all, while in the case of a pipe,
the speech might occur slowly.

Reproducing pianos require close tolerance since the valves operate
over a wide range of pressure and at the low end, may need to respond
at 4 inches water pressure or less.  Some quiet organs, such as the
Welte cottage orchestrions, operate below 4 inches, and thus must have
highly consistent pouch porosity that is closely balanced with the
bleeds.

To logically achieve good performance, one doesn't start adjusting
the bleeds to compensate for pouch leather that doesn't duplicate the
original flexibility and porosity.  In other words, leave the bleeds
alone and fix the pouches.  Testing the replacement pouch leather will
reveal the need for sealing.  Testing should not be abstract, but
actual.

If the valve repeats correctly under the most critical conditions,
who cares how many bubbles some lab test setup makes?  (Of course, if
a person rebuilds a certain system often enough, lab equipment can be
calibrated to give meaningful results which then can be a standard for
pouch porosity.)

Some original pouches always were sealed.  Most notable in that
category is the Ampico B ball bleed valve.  Documentation confirms it,
as well as the residual evidence that rebuilders have readily observed
when taking those valves apart.

In general, pouch sealant should add no measurable additional weight
or stiffness to the pouch.  It certainly should not leave the pouch
100% airtight.  Leather pouches always leak.  They were never intended
to be completely air tight.  Some common sense is required on deciding
"to seal or not to seal".

Most secondary valves don't need sealant.  Traditional sealant is
diluted rubber cement or egg white (both very easy to find).  Anything
else should be considered experimental and unproven (it hasn't lasted
fifty years or more).

There is reason for confusion.  And who is to blame?  Blame it on the
vast array of machine designers if you will   There was no set standard.
Blame it on the machines (knives) that split the hides.  Or if you are
really angry, blame on the stupid animals that contributed their hides,
for their lack of consistency.

Chose high quality replacement pouches to match original thickness.
Check for pin holes.  Check for porosity.  Seal if it was done
originally.  Check the performance, seal as necessary (most of the
time).  Don't experiment.  Enjoy the results.

Bob Taylor