Greetings,  The valve in question sounds like the pallet valve which
opens when the pedal is actuated.  The valve was often connected with
a tee to the signal tube for the number 2 accompaniment dynamic valve.
This causes the number 2 accordion pneumatic to close when the soft
pedal is on so as to "compensate for lost motion" caused by the hammers
being moved by the soft pedal action.

Some pianos had a double pallet valve here and it could be connected to
more than one tube at the same time.  Depending on how your soft pedal
is regulated, and everything else in the piano, you may want to connect
this valve to number 2, numbers 2 and 1 (if you have the double valve),
or to nothing.

The better the overall condition of the piano and the better the
regulation, the less you need in this boost.  Besides, most rolls were
coded with the number 2 on whenever the soft pedal was used so this
valve is largely redundant.

As far as I remember, this valve is mentioned in the original
literature.

Best regards,
Spencer Chase
Garberville, CA
http://www.spencerserolls.com/