Many members have offered suggestions about this, speculating on possible
difficulties in both the primary and the secondary valves.

It seems to me that the first step should be to isolate the problem;
is the difficulty in the main chest or the secondary deck?  My own
inclination would be the secondary deck.  There the valves are arranged
in 2 alternating rows, so some common problem could possibly affect 2
notes separated by one good one, whereas the 3 row arrangement in the
main chest would mean that the problem just happens to occur twice, but
with little possibility of any commonality.  But this is easy to check.

Remove the whole upper deck, down to the point where the primary board
screws onto the rim of the main pouch board, leaving only the main chest
in the piano.  Plug the supply passage for the upper deck and the motor
hose.  Cover the row of holes in the upper edge of the main pouch board
with tape.  Although there are no bleeds for the main pouches in a double-
valve action, there is enough seepage through the pouches that the notes
will work, although too slowly for regular playing.

Poke a few holes in the tape over the channels to the offending main
pouches, and test the action by uncovering the holes with the finger.  If
they work OK, then the problem is upstairs.  By gently sucking or blowing
into these holes with a piece of rubber tubing held over the hole, one
can quickly assess the functionality and repetition of the primary valve
action, by comparison with notes known to be good.

This is also a good way to locate one of the most common Standard
problems, a breakout in the wood between the pouch channels and the
screw holes which pass between them to attach the pouch board to the main
chest.  Depending on whether the breakout leads outside or to the chest
interior, this fault can cause either a cipher or a silent note.  This is
cured (on the spot during the test) by 'lining' the upper end of the
channels on either side of the offending screw holes with a little glued,
rolled-up piece of paper, inserted in the hole and then expanded to the
walls of the hole with a waxed stick of wood, then trimming off with a
razor blade.  If you have some brass tubing which will fit the holes
snugly, it is even easier to fix this by driving a short piece into the
hole, flush with the top of the board.

If the upper deck is the villain, it means that something there is
allowing some vacuum to still get to the primary channel even when the
note should play.  Look for some cross connection between the two
offending primaries.  Don't forget to look for some 'dumb-dumb' fault
(this has happened to all of us) like something on the underside of the
primary cover that prevents the buttons from going up all the way when
the whole thing is assembled.