1999? 6.5L on Sierra 2500 fuel system issues

[GMCnoob2500Sierra6.5]

Hey so my fuel system is not working correctly. I'm having a leak and a fuel line that goes no where?

 

I don't know anything about diesels. Truck was given to me. Here are photos of what's kind of going on. If you can help me identify everything too that would help me research.

 

 

In the pictures I am pointing at a line that goes no where. It's from a box that I don't know what it is. The line comes out of the bottom right corner pointing down. At the box there is a circular thing that I believe could be tightened or loosened.  I believe the line is for extra fuel? Maybe it's supposed to run back to something that goes to the fuel tank. Fuel does come out of it but doesn't leave any puddles on the ground. The truck runs fine after I replaced the return fuel lines on all the (injectors)? But when I am parked on an angle with driver side lower I have the start and stall issue.

 

 

Please help me with any information, I will continue to research but I honestly don't know much about this truck, besides it's a great and powerful truck, good for heavy toes.