Does anyone know what the small fuel line manifold that connects to all six injectors and returns through a check valve to the fuel filter water separator does. I have never seen anything in these posts said about this?

 

Fuel return line.

 

When the pressure in the injection lines gets high enough to pop open the injector and release fuel into the cylinder there is still excess fuel in the injector. In order to keep the amount of excess fuel equal on all injectors it's passed to the small return line. Without the line the engine would run comparable to a gasser with six different spark plugs.