Over the years I have been called out to look at previously restored
Ampico units in which the expression has quit working and/or the piano
plays weakly.  These are restorations that were done up to 15 or more
years ago.

The one thing I seem to find every time is that the seal on Bakelite
valve seats, which are used on the intensity valves and throughout the
system, are broken.  My guess is that at the time the unit was rebuilt
the seals might have been okay or just overlooked, but additional time
and humidity changes takes its toll on the burnt shellac that seals
the seats.  Once the seal is gone, the intensity valves will not work
correctly, as well as other functions.  Most Ampico stacks from the
1920s use Bakelite valve seats.

Leakage though the seat in each valve block can be very slight, but
many together can be a very formidable loss of vacuum.

Brian Thornton - Short Mountain Music Works
Woodbury, TN
http://www.shortmountainmusic.com/