This is a common problem with these instruments.  Yes, the boxes are
often glued down so they don't shift in a move.  You should not have to
pull it up.  You merely need take out the top flanged nipple and find
the flapper.

Often something has lodged in the flapper or often the flapper does not
line up.  I often have to install a new flapper and then I glue a strip
of motor cloth on it to hold it in place at the hinge end.  Then I screw 
back down the flange over it.  The flapper must come up and totally shut
off the hole to the flange to work properly.

Doug L. Bullock
http://thepianoworld.com/