I recognize the problem you are having.  This was such a problem
that I came close to getting out of this business back in the 1990s.
We were having cat fits over leather for valves back in the 1990s and
2000s -- it was nearly impossible to find airtight leather.

If you have a Baldwin piano then I suspect you are dealing with an
Autopiano stack.  If the piano is a House of Baldwin (i.e., Monarch,
Modello, Ellington, etc.) then it is a Manualo stack.  No matter.  When
all leather became chrome tanned, all the leather was suddenly porous.

My predecessor in my shop was a phenomenal rebuilder but he used his
eyes to select leather and bought the lovely suedes he had used for
decades.  The only bad thing is they were suddenly now chrome tanned and
not vegetable tanned.  This was done because vegetable tanned leather
was only lasting 6-10 years before turning into powder.

Chrome leather with 3-4% chromium salts left in the leather was found
to last 50 years so all leather tanneries went to that.  Chrome tanning
has the trait of opening pores in the leather making for soft leather
garments but making "airtight" practically a thing of the past.

I won't go into the trials and tribulations of finding all this out but
suffice to say it took some time to figure out.  I learned I had to go
to the leather warehouse with my testing device and find leather that
was only 5-10 seconds less than masking tape using the same testing
device.  I test for 60 seconds using masking tape.  Most leather tests
only 10-20 seconds and is not acceptable for valve work.  I still do
this to double check all valve leather used in our shop.

One thing that will make your life as a rebuilder better is to
understand the modern leather.  To be airtight, the leather must be
thinner than the original and have a sealed shiny side.  If it is too
thin then glue it to another layer of leather or rubber cloth or to
cardstock and press between two panels of thick glass to glue overnight.

Thick chrome tanned leather while it cannot leak vertically through the
sealed shiny side it does leak horizontally out the edges of the leather
pretty badly.  Thinner airtight leather does not have the pores that
allow leakage through the horizontal axis.  When you glue this leather
down you must scratch or sand through the shiny side but since you are
gluing it, it will still be airtight.

Anyway, I took over the player shop from my predecessor and I found that
I had to make good on his and my 5 year warranty.  Plus I had several
players in the shop for sale and I suddenly knew why I was so unhappy
with how they worked.  I ended up re-rebuilding all the valves in over
a dozen stacks that he had rebuilt for customer or showroom pianos.

I suggest you make a board with a hole the size of that in your valve
seats and add a nipple to the back side of that hole.  Alternatively you
could turn a metal valve seat upside down with a gasket under it and use
that over the hole.  Seal the inside of the chamber under the valve hole
with shellac and use your mouth to suck on the nipple with tape over the
valve hole.  Connect a tube you can use to suck by mouth to make a
suction under the hole.  Once you get to where the chamber under the
hole is sealed airtight put one of your valves over the hole and suck on
the tube.  if you notice even a tiny amount of leakage then multiply
that by 88 and you will understand why the stack is leaking.

The main reason for valve leakage is leaky leather.  The next possible
cause is the leather not being glued down flat.  Whatever valve you are
re-facing _must_ be totally flat or the leather will not seal against
the seats.  Also, if you glue down the leather and do not clamp it down
on a flat surface, then that leather will not dependably seal.  It may
have dried glue boogers under part of the leather.

One problem caused by rebuilders in a hurry is discovered when I pull
out a metal valve seat, sit a valve in place on it and hold it up to the
light in front of me and I can see a sliver of light between the metal
valve seat and the leather in spots.

One cause of this is when the leather is pressed down with glue under
it and is not clamped on a flat surface.  Often the leather on the
dried valve makes a cone shape like a tiny volcano with glue under the
center near the stem and flat only around the edges of the valve disk.

Another cause of this problem is when the rebuilder reused the red fiber
disks under the leather.  These are well known to warp.  I always
replace these 100 year old red phenolic paper disks with modern valve
disks which are dependably 100% flat.

In Manualo stacks and Simplex stacks and a few others, the valve leather
is pouch leather and it is glued down only around the edge.  If your
stack uses this technique, then copy it exactly.  This allows the pouch
leather to not be glued down where it touches the valve seat and the
leather flexes as it is pulled down into the hole in the seat to make a
much better seal.  The leather is not stretchy enough to belly out but
works really well to seal.  If you change these to the standard thicker
leather glued down to the disk then you can make a whole lot of work for
yourself as they don't usually seal as well as the original factory
method.

So plan to pull a few of your valves apart, test them and once you
determine which of these problems is causing your leakage, expect to
once again rebuild the valves on your piano even though they were
previously rebuilt.  Most of what we are doing nowadays is rebuilding
previous rebuilders work.  The cloth has now failed and we get to see
and replace lots of sponge neoprene and patent leather valves and even
poorly installed leather valves that have leaked and played poorly for
decades.

Doug L. Bullock - Piano World Enterprises
Alton, Illinois
http://thepianoworld.com/