Hi all,  thanks to everybody who replied to my posting.  I can rule out
some of the possible causes given:  It's not bad pneumatic cloth, it's
brand new (and tested).  It's not the tubing, also brand new and no
clogging or kinks (the stack is above the keybed).  It's not rat-eaten
pouches; well, at least they aren't eaten, but they might be leaking
too much.

It's not the piano action -- even though it needs work, it works pretty
smoothly.  It's also not the bleed cups, at least not them being sucked
out, as in that case the note would never play (thanks for making me
see that).  The bleed hole would then be bigger than the trackerbar
hole.

So what can be the problem?  Too much bleed, for some reason?  Leaking
pouches or loose bleed cups (probably not the last, but worth checking)?
Too much valve travel, so it won't seat properly?  Debris on the valve
or valve seats?  Or leaking/loose pneumatics?

I know of one that is definitely leaking as I hear it hissing, but
I don't hear the others (it's probably five of six notes).  I'll check
it out, maybe this evening, and make some better pictures of the stack.
I now have some on my web site, but they aren't so great:
  http://members.home.nl/japberkers/Waddinton (mind the capital W).

More about the music rolls slowing down: It's not because of the play
brake removed, then it winds up on the spool loosely and slips, but the
air-motor keeps turning as the rolls slips.  In my case (with very, very
little drag on the roll) it winds up tight enough to prevent slipping,
but it's the air-motor that slows down as if it has too little power.
The air motor is rebuilt twice as the first time I used bad cloth.  But
it's done right now.

When sealing the hose nipple and turning backwards I feel resistance;
I'm afraid of breaking something when I keep turning it.  So the motor
works well, I rebuilt it with help from a professional rebuilder.  But
still it's the case that near the end of the roll the motor slows down
or even stops momentarily in passages with no notes; when I pedal
slower it gets rolling again.  (Can it be the governor collapsing all
the way?  In that case why does it keep turning when in the beginning
of a roll?)

The trackerbar is polished, so that's also not the problem.  (And I
didn't lubricate the brakes.)

I don't see where the problem would be if it's not trackerbar suction.
When in the beginning of a roll this is not a problem as the take-up
spool diameter is not that big then, but as paper winds on it the
diameter gets bigger and bigger, requiring more power to turn it.

When lots of notes play together the roll turns smooth, but with only
one or two playing it will go much slower causing unwanted and
unmusical tempo variations.

I hope I get more troubleshooting tips.  The tips about the soft notes
will be very helpful, I think.

Thanks in advance again,
Niels Berkers
The Netherlands