3 Line mech. fuel pump??

[383drvr]

My '83 has dual tanks, Q-jet carb, and has a 3 Line mechanical fuel pump on the engine. Why in the heck is there a third line? The only thing I can come up with is that its a return line for an optional TBI system.... Any ideas? I'm just trying to simplify everything for when I do my rebuild, and would like to eliminate some of this nonsense....

Also, on a different subject, Is it ok for me to plug the vapor canister line that goes to the tanks? When I removed the canister and plugged that line last time, the tank selector valve split and spewed gas... Its possible that that was a coincidence, because the valve didnt work and was damaged to begin with, Its just that I dont want to go through having to replace it again.... (junkyards suck). Does plugging that line create pressure in the tanks that can crack the valve???

Thanks for listening