Datsunitis,
OK, looked at my car last night. Forgot there were two panels back there (age and crown royal are probably contributing factors). The one closest to the front of the car is access for the fuel sending unit and the on closest to the rear is the vent to the evapo unit. The vaccum valve in the engine bay is emission related. As Tommo said there should be vaccum lines attached to both of the forward pointing ports. On my car they are hooked into nipples on the right side of the brake master cylinder below the resevoirs. The other line I assume goes back to the evapo unit. Couldn't tell you how that works but that is the way my car is set up.
BTW...the bath tub stoppers for cylinder caps is brilliant. I've been wondering what to do about that as OEM caps are no longer available. Do they fit tight enough?
Kelmo