Fuel Starvation, no apparent cause!

Today I have ran the engine with a fuel pressure gauge fitted which showed low pressure. A subsitute mechanical pump has already been tried without success and as the fuel pipes (and filter) have already been entirely replaced I shall look at the tank next.

Could there be insufficient resrtiction in the return line to the tank? On my 912UL there is a snapped-off self-tapping screw used as a restrictor in one of the legs of a 4 way metal splitter that feeds the two carbs.