Hi Bill,
The extra lines on some sending units are return rather than vent lines. It is my understanding that GM used return lines on some carbureted vehicles in order to help prevent vapor lock. They used a special fuel filter or mechanical fuel pump with a return line outlet. That outlet has a small orifice that allows vapor to escape from the fuel line. And any liquid fuel that goes through the orifice gets dumped back into the tank through a return line that connects to the tank through the special sending unit with the return line fitting.
There are 2 types of gas tanks ... with and without EEC (evaporative emissions control). The EEC tanks have built-in vent lines that connect to a liquid/vapor separator mounted behind the rear seat (on 72 & earlier Novas). The 73-up tanks have a built-in liquid/vapor separator and no longer have the part behind the seat. A vapor line runs from the separator up to a charcoal vapor recovery canister in the engine compartment. The canister stores any gas that evaporates from the tank. A purge line connects the canister to the engine so the stored gas vapors can be drawn out. In theory, this system should save a bit of gas but may be difficult to re-install in your car if many of the original parts are missing.
There are also 2 basic types of gas caps. Emissions and non-emissions. The non-emissions caps freely vent the tank to the atmosphere (which can waste gas through evaporation). Emissions gas caps have a built-in pressure relief valve that will only vent to the atmosphere if the pressure gets too high. Both styles of caps provide vacuum relief. The EEC systems require an emissions type gas cap in order to be functional. But you should be able to use an emissions type cap on a non-EEC tank as well. In fact, I think this was done originally on some cars prior to the introduction of the EEC systems. This will basically keep evaporated gas contained in the tank unless the pressure gets too high. It's probably not quite as good as EEC but definitely better than a non-emissions gas cap in terms of reducing gas lost to evaporation.
