95 7-Series Gas Tank

[DeerHunter]

Weirdness with my 740iL- On a trip my fuel pump went out. Service replaces it (in right side saddle tank) and sends me on my way, warning not to let gas guage go under 1/2 tank because the left saddle tank sending unit is also out (doesn't have the part) and if I do car will run out of gas. Now the interesting part. According to my local BMW service tech the earlier 7's (pre-1995) had a left hand sending unit that actually sent fuel to the right hand tank, not so with this car. On this car there is a siphon tube that is venturi operated off of the pressuized fuel line and that's how the tanks even themselves. Interestingly when my wife let the guage go below 1/2 once the car did "run out of gas". I know that it is not using the left hand tank at all because we only get around 220 miles to the 1/2 where we used to get well over 450(full) when everything was running right. My local service tech has NEVER seen this before and says I need a new tank ($1100) because the siphon tube is not repaceable. Question: how did the tech who fixed the pump know that the problem would happen-he predicted the "run out of gas" symtoms and why did all this happen at the same time the fuel pump went out. Weird. Help?