You have a classic case of 'bellows cloth death'.  The cloth, which
came from Player Piano Company, was made with synthetic rubber and has
hardened and is now about crispy.  You should tape up all the ports
from the bellows system, pump the reservoir closed and time how long it
takes to open up -- less than 15-30 seconds is not acceptable.  Some
reservoirs hold for several minutes.

You will probably need to recover the bellows system.  Also, be very
careful to choose flap valve leather that does not seep.  That is not
always easy to do.

It is very likely you will need to recover all the pneumatics, of all
sizes, in the piano.  You may test this by taking, say, the soft treble
pneumatic and wet the motor cloth with a wet rag.  Now blow into the
supply and watch the water make foam through the motor cloth.  When you
blow into the supply and close off the small hose to the pouch it
should be totally airtight.  With or without the water you will notice
whether the cloth is porous or not.

You might try running the electric suction directly into the stack and
crank the roll by hand, just to see what the condition of the stack
really is.  If you put 30" into the stack supply you should be able to
monitor the same or very close pressure (27-29") at one of the tracker
tubes or the tracking pneumatic supply hose.  This would indicate that
your valves are sealing.  You may need to disconnect the tracking
device to check the stack because its motor cloth is most likely leaky
as well.

Good luck.  If you replace valves in the stack watch for seeping
leather.  It is everywhere nowadays.

D.L. Bullock    St. Louis
http://www.thepianoworld.com/