In 160123 MMDigest Harold Ball asked about the Ampico B pump spill
spring and its adjustment.  I would say to remove the spring and add 
some length to it by increasing its length a small amount, say 1/4 inch,
and then see if you get a better expression curve.

A slight variation in the length of the spill spring will make a large
difference in the playing of the piano as the spill response is pretty
much entirely governed by the strength of that spring.  Expand the hook
ends a little, put the spring back in and see what the result is.  You
are looking for a musical effect not so much what reading a vacuum
gauge is giving you.

Hope this helps,

Ben Gottfried - Ben's Player Piano Service
Richfield Springs, New York
http://www.bensplayerservice.com/