The fuel lift pump is integral to the CP3 injection pump - it draws fuel from the tank thru the filter anytime the crankshaft is turning, and can manage this with an empty filter, but may take some time to fill the filter element when cranking the engine - best procedure is to prime the empty filter with the primer pump, then bleed any trapped air, then prime again till the pump is difficult to push, then try starting the engine, as suggested - may take a few cranks to clear the air between the filter and the CP3

That's why some have installed an auxilliary electric lift pump in the system, between the tank and the filter