Hi Gail,

Back pressure will not harm anything as long as it doesn't exceed
20 pounds of pressure. In other words, you could reverse the flow
of your vacuum cleaner and apply the pressure to the stack without
fear of harming anything. Some technicians refer to this as the
"blow test". If everything is working well, the note bellows should
inflate and there should be no 'hissing'. On a good stack, one good
steady blow from your lungs should be enough to 'puff out' all the
bellows for a few seconds. (You do have some loss of air because
of the bleeds, but that can be reduced by putting tape over the
trackerbar during the test.)

Generally, most of the pressure loss that's experienced when running
the test is due to microscopic leaks in the bellows cloth and leakage
at the exhaust valve facings. However, with the early Kimball, if the
blocks are loose, you should be able to hear a whistling or hissing
sound with a listening tube or a stethoscope*. You have to check all
the way around each block, which is why using the pressure from a
'reversed' vacuum cleaner that's located in another room -to reduce
the motor noise- is the best approach. *If you are using a stethoscope,
you must remove the end piece so you have a clear piece of tubing
between the area being tested and your ears. If you are using a
listening tube, which is nothing more than a piece of rubber tubing
that fits into your ear canal, hearing small leaks is quite easy if it
is quiet in the surrounding area. Also, when testing the blocks with
vacuum, put the listening tube right over the exhaust valve port. If
the intake facing is leaking, you'll hear it.

The real problem with troubleshooting the stack with vacuum or
pressure is finding (or creating) an environment where there is little
to no background noise.

Another cause of reduced volume has to do with the regulation of
both the piano action and the player action. If there's as little as
1/8" of lost motion in the piano action and 1/8" lost motion between
the player action and the piano action, the volume of the music is
reduced by 50%. That number gets larger if the let-off adjustment
of the piano action isn't the required 1/8" from the strings.

Here's what you have to remember. The total movement of the note
bellows is only 3/8". If half that distance is wasted compensating for
the lost motion in the piano and player actions, over 50% of the
'work' being done by the bellows is wasted (or lost). One of the most
common jobs I do in older 'modern' players is regulating the piano
and player actions so they meet manufacture specifications. The
reason more than 50% of the power is lost is because a bellow
develops its maximum power at time zero -when it first starts to move.
From that point until the bellows is fully collapsed, the power curve
slopes downward to zero -when the bellows is fully closed.

Naturally, therefore, applying 50% more vacuum will compensate
for the power lost due to poor regulation, but it also causes the
various soft materials to wear out faster -exacerbating the whole
situation, and creating the possibility for damage by 'hitting' the
parts in the piano action too hard.

Lastly, try pulling off each valve block by hand. If they are securely
glued to the deck, you can pull on them as hard as you like and
they won't move. But, if the glue seal is weak, you'll know it...

Musically,

John A Tuttle
Player-Care.com