But before rushing and adding extra pumps (ie adding more to go wrong) one should make sure that one's engine
is not one of the small marine
diesels whose own lift
pump will happily self prime, even with a lift
on the suction side.
As one example, ours self primes with both the primary and secondary filters dry and with around 1-1/2 metres lift on the suction side from the low fuel
level in the tank (our fuel is in the fin keel
so well below the engine).
With respect to drain back from the fuel lines when changing the primary filter this is often negligible or nil. Partly because of their small volume but also depending on the configuration. Using our own case as an example, because it is all down hill from the engine lift pump to the tank top the lines appear to drain back into the tank whenever the engine is stopped as if I remove the primary filter element there appears to be only the spillage dribbles from the bowl/element. In any event, unless there are other than short runs the volume of fuel in the fuel lines of a small diesel
is very little anyway and is easily caught in the container used for drips when removing the filter bowl/element. In those types of situations there is no need for valves about the primary filter and they only add to complexity (generally increased complexity => decreased reliability).
As DavidM says, there should be valves to cut off sources of fuel so connections into the tank should be valved. But in our own case we have not valved the leak off return from the injectors or the tank vent but have reduced the risk there by teeing the leak off into the vent at the tank so there is only one connection rather than the common two (I personally don't like to valve the tank vent due to the opportunity for shutdown of the engine through fuel starvation and subsequent confusion as to the reason - others may feel differently