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?

[79BIG10]

The third line is a return line from the fuel pump. It bleeds off any excess the pump doesn't sent to the carb.

[Earl Filter]

I guess it's been a long time since I've messed with a mechanical fuel pump setup...I didn't realize they had a return. Thanks!

[plumbcut]

im pretty sure that only heavy duty and/or AC trucks had the 3rd (return) line for that year....got any pics of the truck? where are you from?

[Earl Filter]

Originally Posted by plumbcut

im pretty sure that only heavy duty and/or AC trucks had the 3rd (return) line for that year....got any pics of the truck? where are you from?

I just bought the truck from a friend of a friend for $300. It is a C20 3/4 ton, and does have factory A/C. The boxes and compressor mounted in the bed have been there almost since day one. The guy I bought it from bought it from the company he worked for at the time after it did 2 years of service work. He was the driver from new. I'm just west of the Houston, Tx area. Here's a pic of the truck, and one of the SPID sheet: