The spring-controlled bellows operates a valve to control the
accompaniment level when playing Themodist rolls.  If the bypass valves
(either pneumatic or slide valve)  are open, the pump pressure will be
directed under the regulating pneumatics and cause them to "appear" to
not be operating.  To properly set the regulator, make sure all the other
valves are closed first.

We did find one instance where the supply nipple between the expression
unit and the player stack was pushed in far enough to restrict the flow
to the stack.  This was on an Angelus unit.   I hope this helps.

Bob Lemon¶
Lemon's Player Piano Service¶
Sacramento, CA