Tom
Check out the link below. It is a good discussion of all of the components in the vapor recovery system that would have been on your car originally. There are a lot of others, but this one is a good starting point.
Take an inventory of what you've actually got.
Assuming you've got the components, I would start with the under dash cube first, and make sure it is not leaking. I think the referenced thread mentions that most of these crack around the bulls eye in the bottom.
Next replace the "clear" vent hoses that connect the tank, the under dash cube, the filler overflow and the fender tank. Don't bother trying to tighten the clamps. If the hoses have turned that familiar brownish color, they will be too stiff. You will need to carefully remove the clamps and the "Y" fittings and save the old hoses to use as templates for the new ones. Be careful with the "Y" fittings. They will be very brittle, and I'm pretty sure they are NLA. The specs for the replacement tubing is in the thread. Also save and re-use the clamps if possible. They don't make 'em like that any more.
There is a good chance that these two steps will solve the problem. I've been told that the old type canisters are pretty much good forever. It is possible that the braided lines between the canister and the engine are shot, but I would wait to do anything with those until you've done the above.
Good luck.
http://forums.pelicanparts.com/porsche-911-technical-forum/734846-need-little-trunk-help.html