When a pneumatic opens, it generates suction.  The suction level is
obviously less than is required to collapse it, so you can expect a
pneumatic to open more slowly than it closes, unless perchance the
outer valve seat has a larger opening to compensate.  Some do; others
don't.  Any type of helper spring to assist in the pneumatic's opening
will tend to slow the collapse, unless pump suction is high enough to
overcome at all times the greater resistance, especially when the piano
is playing softly with the spill valve passing more air.

If the dampers are properly regulated, they need not lift higher than
an eighth of an inch to obtain the desired effect.  For good measure,
three-sixteenths need not be deemed excessive.  This is considerably
less than the sustaining foot pedal normally lifts them.  It is a
concession to the needs of the player in regard to the automatic pedal
device's tendency to gobble up large amounts of air even when carefully
adjusted.

In non-player grands, there is often little or virtually no lost motion
between the felted back of each key and the damper lever or lifter.
This is not a problem when the action shifter or "una corda" pedal is
used for soft effects, as it is also on some Duo-Art grands.  However,
many grand piano keys (Steinway especially) tend to be heavily weighted
in front, especially towards the bass.  When the pneumatically
activated hammer rail raises the hammers, the keys will correspondingly
drop from their own weight, if not because of soft pedal compensation
such as found on later Ampico stacks.  (I mention only those two
systems because I am uncertain about the construction of Welte
instruments.)  Any resulting upward pressure on the damper levers may
very well interfere with damping, whenever the hammer rail is
activated.  Piano keys, like the foot pedal, tend to raise the dampers
higher than necessary.  So in a reproducing instrument, we must part
somewhat with traditional methods of action regulation.  Some lost
motion is necessary to enable the hammer rail to function without 
affecting the dampers.  Equally important, the hammer rail must not
lift so high as to require excessive lost motion.  When dealing with
opposing forces, extremes must be avoided.

Jeffrey Wood