1968 GMC C2500 Vapor Canister

[gmc396]

I have a question about the Vapor Canister on my 396 GMC. I've attached a pic below. The large nipple on the canister attaches to the intake manifold. The small one goes to a line that goes through the firewall. I can't easily see where it goes from there. Where does it go and what does it connect to?

I don't understand the function of this canister as there is no line going back to the gas tank; just somewhere behind the dash board. Could I just block this vacuum line at the manifold?

I'd appreciate an explanation.