The recent postings about the Pistonola have excited some correspondence
here, and David Perry's remarks about materials reminded me of a
feature I identified soon after contributing my MMD articles in 2000 --
and found rather surprising.

A cursory examination of the mechanism and the usual descriptions of
its operation suggest that it is the secondary piston which operates
the ball valve controlling the playing piston.

However, closer examination revealed that the lower ball valve seat
and the secondary cylinder were manufactured to the same diameter.
In addition, the downward atmospheric pressure on the ball is always
greater than the upward pressure of the low vacuum which raises the
piston.  And the piston is necessarily a free fit in the bore.  So the
upward force on the piston can never equal, let alone be greater, than
the downward force of the ball on its lower seat: the piston on its
own cannot lift the ball valve!

I confirmed this is indeed so when I temporarily eliminated the
rubbercloth disk from the well beneath a secondary piston.  The thin,
close-fitting rubbercloth disk is not simply a cushion for the piston
but in fact constitutes a larger diameter floating pouch which apparently
seals sufficiently in its well to provide easily the necessary lifting
force.  The secondary piston is really just a convenient-sized rod
connecting the pouch to the valve ball.

Tellingly, D Miller Wilson did betray this, stating (Keith Pritchett's
article): "this valve consisting of washer discs, plunger and ball is
called the secondary valve", and "The air lifts ball off its seating by
means of washer and plunger..."

My guess is that this configuration proved more efficient and economic
than a sufficiently large secondary piston alone.  It also explains
why the bias vacuum to lower the ball valve is not provided 'round the
peripheries of either the secondary piston or the rubbercloth disk, but
from a high vacuum gallery (in the gasket, or the top of the playing
cylinder block in some actions) through the primary valve pin bottom
guide holes.  These are larger (minus pin) and thus easier and more
economic to manufacture than additional minute passive bleed holes, and
more reliable, because they self-clean to an extent and can be cleared
by merely lifting out and reinserting the primary valve.

All most ingenious!

Patrick Handscombe
Wivenhoe, Essex, UK