I do not have an answer for you but in my 87 I have always filled the tank from inside the van[ through the top loading cabinet] I never trusted the outside hookup as I have seen too much side rot from leaky tank hoses on other vans. The 2 thirds factor may[ long shot] be where pressure is enough to put strain on bad hose. Fill it up, if it leaks ,quickly unscrew cap under vehicle and curse me. If it stays, continue to fill from the inside. I aint no mechanic but I have a crazy relation with these things.