John, the IMI Cassette Converter prototype was originally built with
the Reisner valves you described, and was thus quite bulky.  The
production units featured a tiny magnet valve about 1/2-inch square
by 2.5 inches long, with a spring-loaded rubber-sealed armature of
about 0.016 steel only 0.150 diameter -- very low mass.

It was capable of very high repetition rates, and no degradation in
piano performance was observed. With a 15-volt power supply the valve
was good for 40-inches vacuum in an Ampico B.  (The valve failed to
open in a DuoArt when the crash valve drove the pressure to 70 inches!)

The small size was the only reason it was used instead of the Reisner
valve, even though it cost considerably more.  Technician Randy Cox and
I priced the components for a new production run, and the cost was
about the same as your design: more than $6 each in a run of 1000
units.  Too expensive for us....

What's the performance of your design like?  I'm skeptical about
repetition rate if you're moving a massive solenoid slug around.

Keep the ideas flowing!

-- Robbie Rhodes