The last item to check is the gas door lock actuator. This actuator is up inside the passenger side rear fender. It is directly on the other side of the panel pictured, behind the trunk spring. It's held in place by the two screws shown.
This actuator comes in two styles. The one on the car being tested only has a lock side; it uses a spring mechanism to return it to the unlocked state when vacuum is lost on the lock side. Another design acts just like the trunk lock actuator, in which there is a lock and unlock side.
On the spring-loaded lock only style, remove the single vacuum connection from the bottom of the actuator and pull a 10 to 15Hg vacuum on it and watch for leaks. On the lock/unlock style, treat it the same as the trunk lock actuator by testing the lock side since that is where the original leak was found at the beginning.