Where's my vacuum gone? (Longish)

[Lib596@xxxxxxx]

Here is a technical question I hope someone can help me with.

I am always trying to improve the way my dual fuel system (LPG/Gasoline) 
operates.
For best performance on both fuels I have made a vacuum operated gate which 
closes to restrict the air flow  through the air cleaner when operating on 
LPG but stays open for gasoline. This system works like a charm on my Caddy 
but there is hitch with the Imperial  (413 engine). There is not enough 
vacuum generated while I am cranking the engine to operate the vacuum motor 
that closes the restictor. 

So far I have tried taking the vacuum off every different source on the 
manifold and carb. I have closed off all other vacuum take offs and just 
connected the restrictor motor. I have checked for inlet manifold leaks, I 
have fitted a non-return valve in the line to the restrictor. In addition, I 
can't use the vacuum reservoir because it still won't hold a vacuum 
indefinitly and besides, each time I switch to gasoline or turn off the 
engine, the vacuum solenoid would dump the reservoir vacuum. Whichever way I 
try, the vacuum motor still won't operate unless the engine is already running

I know I could just use a cable to close the gate, but the idea is to have a 
completely automatic system with no extra levers, buttons or switches inside 
the car. I could also try to find a 12 volt vacuum pump, if such a thing 
exists, just for starting the car but that seems unnecessarily complex.

Is there a reason why the cranking vacuum is so weak on these engines? Is 
there a way to increase it?

Hope someone has some ideas, I'm right out

All the best,

Tony V.