Do the DELPHI Fuel Lift Pumps have a one way valve or not?

HI
I have had a half empty fuel filter in the morning. Man at 'LR' garage said that the lift pump is failing and that the non return valve in it not working in fuel lift? How can a non return not work??
The supplier off ebay said that the Dlephi doesn't have a one way valve and that is a sealed system on these and check for air leaks in the flow pipe which was my original intention.

is the fuel pipe 8mm?

what size are the injector leak off pipes?

they do have 2 oneway valves,otherwise it wouldnt pump ,they get stuck open ,they are mostly sealed uniuts now ,just get a new one

it was full this morning though??! may just monitor it. car points down a little slope on driveway