I realize that this is a completely unrelated car and possibly system, but might give you a new way of looking at what you have. I was just working on an old R2 (supercharged) Studebaker yesterday and on most of the '63-64 performance models there was a vacuum-operated dashpot on the front of the carb. How it would work is like this, you adjust the carb with the dashpot removed, then install it and start the engine. The dashpot pulls in immediately when vacuum is applied to it and the end of it should be adjusted so that it is just off the throttle linkage - i.e. the linkage is resting against the hard idle speed stop. But if vacuum drops, as if the car is going to stall or is idling low for some reason, the plunger in the dashpot will kick out and return the engine to idle. I suppose that this setup could also be adjusted to allow for fully closed throttle plates on overrun to save fuel - not sure if that's a good idea or not. would probably make for a lumpy idle as the dashpot would constantly be wavering back and forth. But now I'm not being helpful, I'm just thinking out loud (with my fingers?)
In your specific case, if you're losing vacuum through a vent, I suspect that you need a new vacuum actuator - sounds like a cracked diaphragm.
nate