PVC-E diluted with water also serves to help keep old shellac (or
hide glue) from flaking off valve wells and passages, which can end up
getting stuck between the valve and its seat, and happens according
to Murphy's Law.

Regarding porosity of valve chests of player pianos, imagine that
the size of the bleed holes were designed with that consideration.
Thus in those rebuilds that seal (wood or leather or both), the bleeds
might act differently.

Richard Moody - Piano Tuner Technician
http://www.pnotec.com/