On the inlet to the fuel rail, the banjo bolt has a plastic cap on top of it. Under that cap is a screw. What is the purpose of the piece instead of just using a standard banjo bolt like on the fuel filter?
i`m not sure what it does, but in my manual, the part you describe is called a pulsation damper,if it is the same part i think you mean.
A device that absorbs and releases pulsations in flow produced by a piston or diaphragm pump. Consists of a chamber with air trapped within it or a length of flexible tube.