I was working on a Sting II recently.  It would not play at all
with the motor.  Pumping the pedals with the motor running would play
some notes.  There was plenty of vacuum in the reservoir because it
was staying closed.  I adjusted the slider valve on top of the pump
and got it to play weakly, but more than four long, sustained notes
caused it to run out of air.

I put a test point in the main stack hose above the stack cut-off
valve.  It was maxing out at 20 inches and jumping down when notes
played, but never much below 15 inches.  This seemed like more than
enough vacuum, I thought.  It acted as if it had a combination of a
constriction in the supply hose with notes that leaked severely when
playing (I could not easily check the cloth on the pneumatics), but
I could hear no sustained hissing near the valves or pneumatics as it
played.

The reservoir springs are very strong and the reservoir is fully
collapsed all the while.  The governor pneumatic is also fully
collapsed as much as I might strengthen the spring.  Has anyone else
encountered this?

Joseph Osborne - Osborne Keyboard Restoration
Carlisle, Pennsylvania