extended fuel tanks

check that you breathers are not blocked. If they are this will cause fuel starvation from the vacuum cause by the fuel transfer.
Cheers
Wayne-o

Don't understand what you mean by gravity feed. If it is gravity feed, the aux tank has to be higher than the main.
When I had an auxilary tank, it was first set up to transfer fuel under vacumn, i.e. syphon, but you need a well sealed main tank for that to work. Had to put a fully sealing cap on the main tank filler for it to work.
Was not always reliable, even a small air leak stopped it from working, so switched over to a electric pump system manually controlled from the dash. That way i could run the main down and keep the aux as reserve.

I actually bought a new fuel cap to make sure there wasn't a small vent hole I couldn't see.
It did work fine for about 3 years.
After about 50k, you should hear a rush of air taking off the main tank cap if all OK.
Also, don't take the main tank cap off until ready to refill or it will take a little while for the syphon to start working again.
Found the fuel pump worked well. Took 30 minutes to transfer 70 litres across from the aux. Main is only 135 litres on mine, so had to make sure the main is 1/2 empty for a full transfer.
Did it also so I could keep the aux in reserve in case of a problem with the main tank. Could swap hoses around under the car if I had to.
Pump was put on the 85 paj before the cape trip for something a little more reliable. Now doing service in my current 96 model sitting besides the extra fuel filter to help to stop fuel starvation. So it has lasted 15 years and about 300,000k. No, pumps are not perfect, can have thier own problems too. Also an intial cost!

went home and checked it and as far as I can see there is no breather in the aux cap (hence the whole system is air tight so won't syphon). I have passed this onto TJM today who get the car back. As he said on the phone though, the breather could be elsewhere other than the cap.
Anyway will wait and see what they say today.