If the vacuum line is connected and receives vacuum, there may be a leak in the diaphragm in the vacuum pod. The vacuum line T connector itself has a restrictor in it(should be a red band or dot of paint on the rubber T fitting connected to the pod), I think to prevent the flap from slamming open and shut and annoying the vehicle occupant. The problem with the restrictor is that it limits the flow out of the pod such that even a small leak will prevent the flap from fully opening--explaining the partial opening you may have. If you remove the glove box, you can reach in and remove the T connector and remove the small sintered metal restrictor from the T and maybe it will work. But there will be a constant vacuum leak in your system which is not necessarily a good thing.
The diaphragm can be replaced without removing the dash(George Murphy sells replacement diaphragms) if you remove the glove box and the two center vents, the rubber boot that connects to the vents, and the temperature probe that sticks down from the dash. Reach in to the pod from the glove box opening with a long thin screwdriver(flashlight through one of the vent openings) and you can release at least two of the four plastic clips holding the pod together--the spring in the pod should then force off the cap. Keep the cap connected to the vacuum line so you do not lose it during this process. Disconnect the actuating lever from the damper and remove it along with the diaphragm from the pod base. Replace the diaphragm with a new one and reinstall it in the pod base. Be careful not to damage the plastic mounting pins holding the base of the pod in place.
The hard part is snapping the pod cover and spring back on the pod base because the force of compressing the spring against the diaphragm distorts it and makes it difficult if not impossible to seat the top of the pod correctly. To avoid this problem, get some small diameter thread or nylon fishing line and tie the spring into the pod cover--use maybe four pieces of thread tied evenly spaced around the spring and run the tag ends through the vacuum opening of the pod. Pull the tag ends until the spring is fully compressed in the top of the pod and stick a round toothpick in the vacuum opening to hold the tag ends in place. Clip off any excess thread beyond the toothpick. You can now feel the pod cover as it seats on the circumference of the diaphragm and you ease it into the base--practice a few times without the spring and without actually snapping it in place to get a feel for it. Once you get it snapped in place with the spring installed, pull out the toothpick and the spring will be released.
This method worked for me on an 83 300TD.
Peter Ferguson