Hi Robbie and Robert,  Since a rotary piano pump can suck 80 to 100 or
more inches water column, with no flow, it is indeed impractical to make
a manometer long enough to never be sucked out in any circumstances.
20 or 30 inches is enough for most player adjustments.

Making a water trap for a manometer is simple in concept; one just
needs some sort of sealed chamber in the tube from the piano to the
manometer, where the water will end up, if the manometer is sucked too
hard.  The attached picture shows one way to do it.  The details are
unimportant, as long as one keeps in mind some basic principals:

The water must fall into the trap, rather than going all the way into
the piano.

The chamber must hold all the water in the tube, with plenty to spare.
A 30" x 1/4" manometer holds less than two ounces of water.  But try it
first to make sure the bottle is big enough!

Sucking out the manometer is bound to happen.  So it should be easy to
unhitch the chamber; and lift it and tilt it so the water will run back
into the tube when this happens.

When this occurs, some of the fluid will stick to the tubing and the
bottle, and the manometer zero will end up 'lower' on the scale.  A
sliding scale makes it easy to 're-zero' the reading when this happens.

Robbie, if you can't find the old article about this, use the attached
picture.

Best regards,

Richard Vance

 [ Thanks, Richard; I'll soon place the article and illustration at
 [ the MMD Tech site,  http://mmd.foxtail.com/Tech/  -- Robbie