Hi All,  Understanding a little bit about the theory of operation
of a player piano helps most people comprehend the problems they are
experiencing with the mechanism.

Leaky stacks are never the result of leaky pouches.  In fact, the
pouches could be removed from the stack and the leakage factor would
be virtually identical to that of a stack that had pouches which were
100% air-tight.  As it applies to the pouches, the only 'escape route'
for the vacuum is the signal tube which leads to the trackerbar.  And,
as long as the trackerbar is covered with a music roll, that escape
route is effectively closed.

Stack leakage is most often caused by leaky valves.  More often than
not, there is also a valve stem involved.  Most simply put, what
usually happens is that the air seal between the intake valve facing
and the valve stem deteriorates due primarily to corrosion.  In units
that do not have a valve facing with a hole in the middle (and a valve
stem passing through that hole), valve leakage is normally the result
of a poorly seating intake valve facing.  Also, in some cases it is
found that the intake valve seats are loose and leaking.  I refer to
this type of leakage as "static" leakage because it is present when
no notes are being played.

"Active" leakage involves a host of other components, including the
exhaust valve facing and exhaust valve seat, the air passage from the
valve chamber to the striker pneumatic, the seal between the tier and
the striker pneumatic, the wooden parts of the striker pneumatic, and
lastly the pneumatic cloth.

Getting back to the pouches, if they are leaking, the indication will
be that notes are reluctant to activate or they will not activate at
all when a perforation opens a note hole in the trackerbar.  This
happens because the atmosphere, which enters the pouch well via the
open hole in the trackerbar and inflates the pouch, is leaking into
the vacuum chamber (above the pouch), too fast for the pouch to develop
enough energy to 'push' the valve off its seat.

Musically,
John A Tuttle
Player-Care.com
Brick, New Jersey, USA