I wholeheartedly agree with Ben Gottfried that Duo-Art cross-valve
plates should not be replaced.  The stainless steel replacements
sold by Durrell Armstrong have a round opening 1/2" across; the
corresponding opening in the original lower round plates for the
later Duo-Arts is only 3/8" and is designed to work with smaller
round valves.

It is true that the upper plates have a slightly larger opening, but
this is because as a pneumatic opens, it generates less suction than is
required to collapse it, so this variation permits faster repetition,
though not found in all player actions.  Bear in mind, too, that late
Duo-Art stacks, originally built with round-opening valve plates, all
have smaller pneumatics, whether graduated or not.

Aeolian unfortunately used corrosive leather valve facings in some of
its stacks.  In these instances, small unevenly distributed lumps of a
waxy greenish substance (verdigris) will appear on the mating surfaces,
unseating each affected valve enough so that even a few will add up to
a major loss of suction.

In restoring such a stack, I have found it best not to remove valve
plates sealed to the wood with the original burnt shellac unless it
is flaking off with pieces threatening to become lodged in a valve
somewhere.  The plates, if nickel-plated brass, can be cleaned in place
with mineral spirits or denatured alcohol sparingly applied to a cloth.
In manufacturing these plates, the cross area is machined after
stamping, so further warpage should not be enough to worry about.
Resurfaced plates should be replated as well.

Valve leather can be brushed with a fine stiff brush, but the
problem may re-appear in several years.  If releathering is preferred,
I wouldn't know what to recommend; leather quality just isn't as it
used to be 80 to 100 years ago.  I would want to clamp samples from a
full skin to clean brass, place them in a damp basement and wait months
or years to see what does or doesn't happen!  Not very practical in
this day and age...!

Access to the valves is gained through the top plate which is sealed
to the wood with a blotter paper gasket, which may be re-used if not
damaged provided that the gasket, plate, guide and screws all be
placed in none other than their original positions.  If gaskets are
replaced, thicker material must be avoided.

Valve travel must not exceed .040"; I don't know how much lower you
can go, considering that the upright cross-valve stack has smaller
pneumatics.

Jeffrey R. Wood