I, too, live in an old house and have had problems with humidity for
many years.  Too much humidity in summer, too little in winter.

While I do not have problems with shrinkage, or leakage in my pianos,
I have problems with tuning stability during extremely cold weather,
and increased indoor heat at these times.  I tried using a room
humidifier, and it was not sufficient for the pianos.  In addition,
old houses cannot take extra humidity in winter. I had problems with
condensation on the second floor rooms that cannot be heated.  The
sheet rock ceiling in the upstairs hallway began to buckle and fall!

I am not in favor of devices designed to regulate humidity inside or
under pianos.  My solution was to purchase a large piece of plastic
sheeting (painter's drop cloth type) and cover the entire piano with
it, so that it touches the floor on all sides.  On the floor, under
the plastic, I have placed two large flat roasting pans and have them
filled with water.

Also, under the plastic I have placed a hygrometer on the top of the
piano, and I read and monitor it so that the humidity under the plastic
does not become too high.  If too high I can add or subtract the pans
of water.  At the moment it reads 34%, which is ideal and much better
than 18% inside the house.  This has immensely helped the tuning
stability and I am sure it will protect the sound board from extreme
dryness.  I would not recommend exceeding this percentage.

Another thought about David Schroeder's problem with low humidity.
Sometimes the pouch leather will shrink slightly during periods of
low humidity.  When the pouches shrink they can depress the valve
very slightly, causing leaks.

There are a couple of things that you might try.  With the tracker
bar sealed off with tape, remove the hose to the stack, and apply
strong vacuum from a vacuum cleaner to the stack for about five
minutes.  This higher vacuum, might stretch the pouches enough so
the valves will not "riding" the pouches.

The other solution would be to remove the pouch board, and massage
the pouches to stretch them slightly.

Or this may not be the cause, but worth a try.

Bruce Clark