The computer and vacuum switching valves controlled the canister. It is important as the others noted for capturing the expanding gas that comes off your gas tank on a hot day. It also provides the vent fro the gas tank to replace displaced volumes of gas if your gas cap still seals. The gas tank it self has three vent lines on top strategically placed so the tank will vent at any angle.
Do you still have the hardline crossing the top of the engine bay? I just recently re-installed all this stuff. I made a new hardline to cross the engine bay from fuel line. Do you still have the vacuum switching valves at all? The computer relied upon temp and speed inputs to allow the canister to vent, usually in tandem with the throttle positioner. If you have a weber and most of this stripped than you have lost the factory inputs.
I found a simple solution, and thus far it has been working perfectly, as near as I can tell. Get a Delphi CV10039 or equivalent from 1990's to 2000's GM vehicle. It is a simple vacuum actuated valve. I have noticed a lot of similar type valves in charcoal systems now. Sometimes it is integrated right in the top of the can. It usually has three ports, two for the larger vent line, one smaller that triggers it.
I used 5/16 fuel line, fit the larger ports fine, ran it from the canister to the valve. I teed into the PCV line. This may take some adapters, really it just needs to go into the intake manifold someplace. The small fitting needs to go to a ported vacuum source. I teed into the vac advance for the distributor. This opens the diaphragm in the valve only when the throttle is open. The idea here is that the vapors from the canister can throw off idle, so you do not want to draw them in all the time. Also having the canister directly vented to the manifold will actually allow it to just vent back out the aircleaner all night. The vacuum valve allows it to vent only when you are giving throttle. This how GM did it, and others and mimics the Toyota system.
Like I said, it has worked for me the last couple months. Been parking in my garage every night, and no gas smells in the morning, tank seems to be venting fine, so I think it is a viable solution that can be done easily on a desmogged rig.
