Fuel return non return valve

[JeffRoo]

Hi all, just a quick question for my understanding. I replaced the fuels lines yesterday for E10 fuel and there wasn’t a non return valve fitted in the return line. I’ve now fitted one, but what is it for? Obviously it ensures flow in 1 direction but I’m after the actual reason it’s fitted. 
van is a 84 velle with 1.9dg pierburg carb. 
thanks 

[Aidan]

I'm figuring it is to stop backflow pressure into the carb when climbing a steep hill with a full tank of fuel when the head of fuel could potentially exert too much pressure to allow the return to flow; the fuel pump is running constantly, though flow rate is determined by revs as pump is driven by the engine (with original mechanical diaphragm pump, electric fuel pumps run at constant pressure), and the return allows excess fuel in the carb reservoir to escape back to the tank so keeping pressure in the fuel lines below a certain level so reducing the chances of fuel line rupture or blowing off spigots and maintains fairly even pressure within the carb so fuel flow through the jets is within the design parameters

[bigherb]

It is to stop fuel syphoning out of the tank when parked uphill.