Charles Schubert used the word "hacker" to describe some person who he
believed used PVC-E glue to seal valve caps.  I am not singling Charles
out in my response as I believe he is in the majority opinion on this
one.

Clearly the use of PVC-E glue is not a historically accurate choice,
so that is one factor working against it.  PVC-E is not as reversible
or undoable as burnt shellac which is something also to be factored in
to any decision as to whether or not to use it.

However, PVC-E glue is Very much reversible.  PVC-E glue is water
based, heat and steam and not all that much extra effort will take the
stuff off.  As far as functionality goes (holding up for a longer time
and working well), I believe PVC-E is a superior choice for sealing
valve caps over shellac in a few instances.  Shellac originally used
on upside down Ampico A valve caps failed, it dried up/became brittle,
shrunk and cracked creating leaks through those porous cardboard
spacers used to adjust the valve travel.  Many player pianos have
stopped working over time or certain notes stay down all the time
because chunks of shellac from valve caps broke off and fell into
the valves.

For sealing other valve caps I've used burnt shellac almost always.
I get my shellac flakes from Liberon they are supposed to be very high
quality.  I mix the shellac flakes with alcohol a thinner viscosity
than I need.  After the shellac flakes dissolve I burn it until I get
the desired consistency.  I was taught to always burn the shellac and
not just mix it thick.  I was told that burnt shellac will have better
adhesive properties and will hold up longer than unburnt.  I still
don't have confidence in burnt shellac for upside down AMPICO A systems
I use PVC-E glue applied by a syringe.

In the opinion of many, if not most, its not a question of agreeing to
disagree but player rebuilders who use PVC-E and other non traditional
materials on occasion are "hackers" regardless of the situation.  Many
of us are quick to call people hackers, butchers, etc., over valid
differences in opinion.  Many of us are also intolerant and indignant
towards conscientious people with a good aptitude for this work who go
through the normal learning process in rebuilding automatic musical
instruments.  Many unfairly judge and "brand" those well-meaning
conscientious with good common sense who did work on these instruments
at a time when few resources for learning player rebuilding or materials
were available.  In my view, we need to put the use of the label 
"hacker" in its proper context, not over use it.

Bill Maguire