Thank you for the link to the tutorial - I would not have got this far without it.
Here's the pic...

The red circle shows the empty connector.
The yellow circle shows the vac pipe connecting to what I believe is the air cutoff valve solenoid.
If I take the vac pipe off the solenoid and connect it to the empty connector I can hear a whirring sound which sounds like it should be doing something other than sitting there idle :-/
Any comments appreciated.