Before going any farther, take a look at the piano yourself.  It may
not be the fault of the switch.  Perhaps the shut-off pneumatic is
"on" all the time.  That is, perhaps it is trying to shut it off all
the time.

There can be several areas of leaking that can cause this to happen.
If it is truly the switch, good luck.  You may have to hand-craft the
switch lever/button system.  At the same time, take a good look at the
shut-off pneumatic itself.  I find them to be too small, poorly crafted
and now quite leaky.  All and all not very efficient.  Recover it well
now and save yourself a headache.

Jeff Davis - in the Mediterranean climate of Seattle