I am repairing a set of bellows on a German sing-bird box, dating from
around 1910.  I have been following the guidelines set out in Mason's
book.  Briefly, this is the story so far.

The bellows wind chest had been broken in half a long time ago by the
look of the break grain.  To cut a long story short, I fixed it and
stripped all the old crud off, but not down through the varnish to the
wood itself.  I have a feeling this is where I went wrong, but let me
tell you the rest first.

I ensured the whistle valve had clearance and free movement and the
flange surfaces met flush.  I resurfaced the gasket to the whistle, too.
I have cleaned the whistle thoroughly along with the whistle valve and
checked all airways and all has free-play.  I have replaced the chamois
hinges and renewed all the valves and valve braces.  I wrapped the
rebuilt unit using the recommended zephyr skin membrane using paper
stiffeners (all cut to Mason's specifications).

The mechanism itself works fine, but there just doesn't seem to be
enough air pressure getting through to the whistle.  If I manually move
the air reservoir back and forth I get a good note out of the whistle.
Yet, in its animated state, the whistle is just too quiet.  It seems
to pick up the stronger pitches [sustained notes?] fairly well, but the
lighter 'twitters' are lost.

I have rebuilt the bellows twice now, thinking it had to be the new
valve material I was using (100 gsm paper).  I even experimented with
pressed and sealed chamois leather to recreate the original material
used (which I salvaged during deconstruction).  I have also tried
plastic, thinner paper and combinations of all the above.

During tests with these valves I could never seem to get a really good
seal against the wood regardless of the material I used.  This makes me
think it could be the wood, or more accurately the varnish (which seemed
still good so I didn't sand it off).

With all this in mind, my question to you is what should I do to make
the bird song louder?  Please watch my video to see and hear what the
problem is exactly.  It does sound a little louder on the video than
real-life, to be fair:  http://www.youtube.com/watch?v=RJpFXasaYD8 

Thanks in advance for any advice you can give.

Kind regards, Simon Richens