To do a leak check on fuel components, you spray chalk dust on them, and run the engine at high power. Then open it up and check. You must use high pressure fuel, the fuel pressure at cranking speed is not enough.
I can't think of any reason to do it at other times than a new installation.
Depends on the size of the leak.
Had experience of discovering a significant fuel leak from a GE90 FMU (or is it FCU? HMU? I forget
) during a 'wet cycle'. Mark 1 eyeball was good enough at cranking speed. Running an engine at high power is not always an option during a 2hr turnround.
