Commenting on John Phillips request on MMD 071214, I would first
recommend that he return to using a dial micrometer with plunger.
The spring force shouldn't be that great to throw the readings off
provided that the leather is not spongy.  Even if it is, the readings
can be compensated for compression if necessary.

If I understand his request, he has a sustain pedal pneumatic that is
activated by three cross valves.  The three valves operate in parallel.
I have seen this configuration as well as units with two cross valves
operating in parallel.  Both designs usually have a single bleed for
all valves.

The object of the multiple valve design is to give rapid pedal response
to both "on" and "off" signals even though the pedal pneumatic is
rather large.  This is accomplished by small valve travel, .028 inches,
and multiple valves to increase the volume of flow through the valves.

Using a single, larger valve to equal the same volume of air passing
through the combined three valves requires increasing the mass of the
single valve.  The valve movement speed is inversely proportional to
the valve mass, all other things being equal.  By keeping the valves
small, small pouches operating at low pressure can rapidly move the
valve over the short distance, .028 inches, and the pedal operation
can be very snappy even at low vacuum.

The valve travel of .040" as mentioned in the Reblitz book is a general
rule for stack valves.  There is some variation possible.  If my memory
serves me, when Art wrote that book, he collaborated with many rebuilders
and the .040 inches was a consensus of what most of us had encountered
in virgin pianos.  In general, if the travel substantially exceeds that,
the valve fails at lower vacuum levels.  If the valve travel is
substantially less, the pneumatic is starved and moves slowly.

The bottom line: multiple, short travel valves are quick, even at very
low vacuum, and the pneumatic is not starved.  The valve travel could
be reduced to .025 inches with no problem.  Remember, the Theme valves
only move .015 inches because very little air volume is needed through
those valves.

Bob Taylor