Fuel vent 3 to 1 union / 2 way valve

[DustyDave]

I am rebuilding the fuel system on my car and found a 3 to 1 union coming off the fuel tank (1978). I see that it connects the fuel tank top vapor space to the fuel vapor canister. I know that with the tightening emissions standards, this is a way to capture fuel tank fumes and consume them in the engine, rather than venting to the atmosphere. I wanted to ask if anyone knows more details on what this 3 to 1 union does exactly? Fiat manual calls it a vapor/liquid separator, but not sure why that is needed or how it works? and not sure why you need 3 connections to the tank?
Also curious of the 2 way valve on the same line closer to the engine. Seems to me a simple tube would quality as a 2 way valve So not sure what purpose that serves and how it functions.
I would like to understand more so I can test to see if the one's I have still work? And of course, just for my own curiosity… Any insights would be appreciated.
Thanks,
David

[18Fiatsandcounting]

David, here's how I think this all works. The liquid/vapor separator (the 3 to 1 gadget) does exact that: It collects the fuel vapors that are captured from 3 separate locations in the top of the gas tank and drips any liquid (that is formed by the condensation of these vapors) back down into the gas tank. With liquids now removed, the vapors continue down the center line on that 3 to 1 gadget to the 2-way valve, and from there they go to the charcoal canister. However, the 2-way valve actually has 3 connections: 1) the input, 2) the output, and 3) a connection to the outside atmosphere. The 2-way valve is there to admit air into the system when a vacuum starts building up in the gas tank, like when you are using up the gas as you merrily drive along. Otherwise, your fuel pump would actually be trying to pull gas up to the engine while working against an increasing vacuum in the fuel tank, which doesn't work so well.

I have to admit I don't have a good way to test all this, but if your fuel pump is pumping as it should and you don't smell gas in the trunk when you open the trunk lid, then all is probably OK.

-Bryan