I thought I might suggest a solution I found some years ago to
a problem with a bellows pump in an old Kimball 88-note player.
I had recovered the pump bellows and it still leaked.

I finally determined that the original laminated wood used for
the base of the pump was porous.  I made a copy of the base from
cabinet grade plywood and then the pump would hold a vacuum for
several minutes.  Perhaps something to check?

Jim Cook