Does anyone know if the Servo Diaphragm on the balance tube is supposed to hold vacuum or not? I just pulled the hose & tried sucking through my mouth & the control arm did move (as it should) but it definitely did not hold vacuum. Any advice would be appreciated. Thanks.

I may be wrong, and if so I'm sure I'll be corrected, but you may be referring to the throttle down diaphragm.

That diaphragm is there to slow down the "return to zero" position of the throttle. If memory serves it is to avoid having the engine or carbs (not sure) backfire because of the sudden cut-off of throttle.

FWIW

E