You are probably rebuilding the three-tier horizontal valve type stack
(Story & Clark also built a less common two-tier stack with vertical
valves).  This action gets my vote for being the easiest to rebuild!

Since the valves are accessed by removing the back cover, this must
be very well sealed when re-installed.  Originally blotter paper
(usually blue) was used.  I have always replaced this with high-quality
double-suede leather of uniform thickness.  It is a bit tedious cutting
out the valve chamber holes when forming the gaskets, but it seals very
well when complete.  If this is not well sealed, the valves will leak
in the "on" position.

I have always kept the original valves and replaced the surface with
new leather (I never used the PPCo valves).  Since these valves screw
onto the stem, they have plenty of play so that they can wobble into
place.  If the valves have been secured too tightly, they will have
trouble seating.  This may be the problem.

The old pouches were usually cloth that must be replaced.  It's good to
make sure that they are well sealed, so that the bleed pressure matches
that used for the old rubberized cloth.

Trying to fit a foreign action into the piano would probably be quite
problematic, since so many design changes would probably be required.

Craig Roothoff
Escondido, California