mercedes 270 cdi 2003 how can i fix this?

Does any one have a schematic with flow chart for my vehicles fuel system normal problem drains fuel from rh of tank and only returns to lh side of tank ie runs out of fuel when tank half full new tank units (500 GB£) and new high pressure fuel pump no results I dont want to keep throwing cash at a problem that Benz are aware of but want to replace whole system to fix with schematics I know i can fix this