The valves on these are made of neoprene, which dries up and warps
creating leakage. If you cover the hole in the top of these plastic
valve blocks with your finger, the pneumatic will close. Sometimes
the problem is at the action cut off valve in the bottom or that slide
valve on top.
The vast majority of the time the main culprit is leaky and stiff
pneumatic cloth. I have found that these modern Aeolian actions are
_very_ compact and airtight. The leaky neoprene valve facings are
usually a minor problem as compared to the leaky and stiff pneumatic
cloth. I will often just recover the pneumatics and repair the
worst of the valves and this solves the problem. Some may think
this is meatball surgery on a something that is better off dead, but
it "does the trick" and the customer is satisfied.
The modern Aeolian system makes me think of a Gulbransen and how
compact it is. One time the valves were so bad they needed to be
replaced. I didn't get that job because unit valves cost more than
$20 apiece, times 83 of them. I could also have split the top section
and replaced the facing material, but that would be expensive, too.