Plumbing looks to be tank to valve to water strainer to lift pump to engine filter to injector pump.
Injector pump leak back to engine filter spare inlet, injector leak back to engine filter bleed screw and back to tank, so that operating the lift pump without engine running should bleed engine filter back to the tank it's just a case of knowing when it's full, any leaks on engine filter should show up by diesel leaking as it's on the pressure side so unfound air leaks getting in are likely to be at the water seperator, if you drain that one you will need to bleed although small amounts of air should naturally self bleed the way your tank return is plumbed.