Hello,  30 seconds seems a long time.  Some specification or another
states seven seconds, if I remember correctly.  Most of the leakage
comes from porous wood.  A reservoir opening in 30 seconds will not
make you player pump hard or difficult.  More important for bellows
cloth is the fact that it must last many years and many operations
before breaking down.

I have had problems with the cloth deteriorating within two years,
to the extent that one could see right through it on the corners.
The pianos stopped working completely.  That cloth all came form Hale
(Tuner's Supply Company).  I believe they have been out of business
for a while now.

Craig Brougher also wrote in his book at some stage that the cloth
quality can vary from batch to batch, and that a sample should always
be tested before use.  I seem to remember that he describes a test
pneumatic or something as well.

Kind regards,
Bernt Damm
Sydney