If the stack in David Schroeder's Western Electric nickelodeon is
anything like the ones found in most Seeburgs, here's what's happening.

The medium/hard poplar wood so widely used in the pneumatic action
industry responds, like wood in general, to humidity changes
principally in two dimensions (across the grain), while remaining
surprisingly stable lengthwise.  In fact, many fine clocks have
pendulum rods of hardwood rather than steel because the wood expands
and contracts less in response to temperature changes than does the
steel.

In a typical 2-tier Seeburg stack, the valve board in dry weather
shrinks about equally on both sides.  The pouch board, however,
shrinks mainly only on its inside (where the pouches are), while the
pneumatics, with their grain going at right angles to the pouch board,
prevent a corresponding amount of shrinkage on the outside, to which
they are glued.  As a result, a readily noticeable curvature appears
in the pouch board when viewed from the end.  This curvature normally
disappears when full summertime humidity is restored to a level quite
unachievable by artificial means.

If the gasket at the joint between the two boards is original, you may
expect it to have long since lost its resiliency.  The joint may appear
tight on the outside, but even with the screws freshly tightened, air
will enter the chamber of any valve not in use and then out of the
channel leading to its pneumatic, passing between the valve board
surface and the compressed non-resilient gasket leather.  Even replaced
gasket leather will lose its resiliency after a few seasons and cause
the same trouble.

To prevent this problem from re-occurring, the stack must be taken out
in midsummer and all the pneumatics removed.  Then, after appropriate
sanding and cleanup, the pneumatics must be re-glued (using hot hide
glue, of course) allowing _no glue_ to get behind where the air
channels join by more than about half an inch.

Of course there will be other sources of seepage under extreme low
humidity conditions.  All packed joints will need to be tightened up,
with re-gasketing where appropriate.  Valves may be suffering from
seepage around the brass seats where they are joined to the wood,
and especially where corrosive leather and the brass are in contact.

Seeburgs are built to tolerate a certain amount of seepage fairly well,
but most rebuilders overlook the major design flaw described above.

Jeffrey R. Wood