Steve Marx asks for advice on rebuilding a Hupfeld player action that
is suffering from leaks.  Steve does not tell us which design of stack
he is working on.  Hupfeld used many different designs during their
very long involvement with automatic pianos, including the later 'unit
valve' design where each note is operated by a single stage valve built
into a cast metal housing clamped onto the stack board.

It is impossible to give detailed advice to Steve without seeing and
personally checking the stack, but the general rule of good restoration,
given the age of the stack, is to do a full rebuild and replace all
wearing and perishable items.

As chance would have it, I currently have a Hupfeld stack on my workbench
and am replacing all leather valve seals and stack gaskets, plus felts.
Doing anything less risks a less than perfect performance and a
possible future strip-down.

With best wishes,
Nicholas Simons, GB