it is important that a non return valve is fitted and system pressure is maintained when the pump is off, it will prolong pump life and aid fast starting of the engine.
Basic rule to always remember bosh supplied it with a non return valve was it
a} they had a huge amount of non return valves they needed to get rid off
b}the designer engineers don't know what they are doing and are actually a bunch of refugees
c}the fundamental design of the pump and it's intended use requires the said non return valve

Gareth it is a gravity fed pump with a non return valve and must be installed with a positive head of pressure and a non return valve