The small pneumatic on top of the spring pneumatic is connected
to one of the two unit valves that activate the sustaining pedal.
There are two of these pneumatics, one for treble and one for
bass.  They are tee'd together and receive vacuum from a hole
drilled into the top of one of the valves.

It is very important that there is no leakage in these small
pneumatics -- test them!  I tested mine, by submerging them in
water.  If there is leakage the water method will show exactly
where the leakage is located.

Bruce Clark