Petrol leaking from old dis-used petrol filler pipe in my LPG Prado

Hi PradOz. I have the same model, but petrol only. As well as the two filler pipes, the 90 series has an interconnecting pipe between the two tanks. It sounds to me as though the interconnecting pipe is still in place in on your vehicle, allowing the fuel levels to equalise to some degree (as it was originally supposed to do, to keep both pumps lubricated).
If I am right, the obvious thing to do is to block off the equalising pipe.

Hi there. I have checked my workshop manual. There is no separate equalisation pipe. Sorry.
The supply pipe runs from the main tank to the changeover valve under the second tank, so this suggests there is a permanent bypass in the valve body to allow equalisation. Looking at the pictures, there are THREE pipes that must be blocked off where they enter the original main tank, that's the supply pipe, the vapour pipe and the return pipe. You would have to hope that the fuel system is properly vented back to the remaining fuel tank, and that any fuel return is also routed to the remaining tank.
Incidentally, I was made very much aware of the situation at Christmas, when I fulled the second tank first, and managed to refuel BOTH tanks via the second filler. I was wondering why it was so slow, and then I got suspicious when the second tank seemed to be taking over 100 litres!!! So there definitely IS an interconnection whch bypasses the changeover valve.
I hope this helps.

I had exactly the same issue and so does my neighbour up the street and anothe bloke I know, set up the same as yours, mine had a yellow cap on the end of the main tank filling hose with a clamp. When I rang up and asked why I literally had fuel leaking onto my garage floor and I mean not just a drip I mean dripping quickly like a tap not turned off properly. My reply was DONT fill the reserve tank to full only 3/4. This allows for expansion in the sub tank. I said you are kidding me arn't you ?? He said no and every 90 series prado is fitted the same way. The fuel will pressurise in the reserve tank and come up the filler neck and go accross into the main tank hose causing the drip. He was right as when I filled it to 3/4 no issues, fill it full to first click of pump and it would leak. I even rand the RTA to see if this was correct and they didnt want to know about it, go back to installer, when it did leak I would get fumes coming up into cabin, absolutely stunk, I pulled the inner trim off on the inside where the filler neck is, I could see the light and ground, the pipe that runs from the gas connection runs down in through the cabin and back out again, what a bodgy job, hole was twice the size then needed, and all jagged edges, no paint bare metal. Not happy jan, so I got some rust proofing paint put that on, then got some silastic and filled in the hole around the pipe, then sold the vehicle. Sorry but never again for me LPG, I wrote on here a while ago what a great job they did, as usual what you dont see you dont know about. I rang him up and told him how dissapointed I was in his workmanship, he told me to bring it back and he would fix, told him I have fixed the hole issue and would not be recomending him to anyone. It was a person on here that recomended him but that not their fault, maybe they got a good job. Regards Steve