My book "Player Piano Servicing and Rebuilding" does include the
statement that key striking pneumatics should be able to fall shut
under their own weight.  When I wrote that in 1985, I was being too
idealistic.  By then, my employees and I had covered many thousands
of pneumatics, so I should have known better than to generalize.
Perhaps some of the largest pneumatics with the most flexible hinges
and thinnest pneumatic cloth came close to that ideal, but it was an
overstatement.

It would be more accurate to say that it should take _minimal effort_
to close them if covered neatly.  I understand that "minimal effort"
is a subjective term, but I've never measured how much force it should
take to close a given size of pneumatic.  As others have correctly
pointed out in this forum, the more uniform the pneumatics are, the
better.

Art Reblitz
http://www.reblitzrestorations.com/