fuel line identification on dual tank system

[Earl Filter]

I took the passenger side tank off my '77 C20 and cleaned it up. This is a dual tank truck, but only the passenger side is hooked up (I got it that way). Off of the fuel sender/pickup on top of the tank there were 3 lines:

The big, fat fuel line to feed the 454
The middle one was about 3 inches long, and had a plug/vent thingy on the end
The third is the smallest of the 3, and went to a hard metal line on the frame that went up to the front of the vehicle.

I couldn't trace the third line, as it was getting dark, but it went up to about the right front control arm area, then went somewhere I couldn't see. What is that line?