Hi Doug,  As you indicated, the check valve flapper was positioned
under the top flanged nipple and held in place with a piece of motor
cloth.  Although the flapper was aligned properly inside the hole
in the top of the box, the flanged nipple was not aligned -- it was
mounted off-center and therefore, even though the flapper would engage
while pumping the piano, it could not fully seal off due to the
misalignment between the two.

Another quick question: When positioned over the hole the motor cloth
is not stiff enough to hold the flapper straight out; instead, the
flapper droops down and the far edge of the flapper tends to rest on
the vacuum screen inside.  Is this typical or should I cut the hinge
out of a little firmer piece of cloth?

Stan Morton