return line on injection pump

[Jim P.]

My son swapped a 1.6 diesel into  a little 4 wheel drive vehicle and we have a question on the fuel return line. When it runs it leaks fuel  from a line on the pump that is tied to the injector returns. We returned that line to the tank, no problems there. But there is another line on the pump that also looks like a return line but no fuel comes out of it when it runs. What is this line for?

[ORCoaster]

Jim P., is it the same diameter as that line that goes to the injectors?  Like 3.5 mm diameter?  Is it on the top of a funny looking part on the very top of the pump, the LDA?  Is this a TD Pump?  If so it is where you connect the boost line from the intake manifold to gain extra fuel when boosting. 

Picture would really help us here.