Belly Tank Overflow

So the belly tank is not in the fuel path, I take it. As you have it plumbed now, the outlet from the fuel pump is disconnected (which has a check valve), while the filler tube from the main remains in place - so you CAN'T use the fuel in the belly tank as it stands now. When you reconnected the OEM fuel line to the petcock you removed the belly tank vent connected at the OEM petcock outlet. Your only vent now is the OEM one from the main with the nipple and small hose at the back of the tank. Though if it's not kinked your overflow should have gone out that tube. And your belly tank's vent sits open in the area of the petcock. Seems gas would exit from that tube from gravity if you didn't plug it. I'm guessing you did the R&M recommended roadside reconfigure they say to recover from a failed fuel pump. Which is to reconnect the OEM fuel line at the petcock after disconnecting the belly vent from there. If you plugged the belly vent - gotta think you had to - then any fuel expansion in the system would come out the OEM vent unless it's kinked - in which case, you would get what you describe if the fuel expanded.

Liquids don't compress/expand except fuel does some from heat. However if there was any space for air in your lower tank plumbing it would expand when you went up in altitude and displace some fuel. You shoulda been there when the wifey was going to make sandwiches for the kids as we were going over Loveland Pass not long after we moved out here. I was watching to see her get a face full of mustard when she opened the very pressurized squeeze container at about 11,500 ft. Yep, that was pretty funny - esp when you see it coming.

BTW, I had the same problem during our return from Inzane. But the cause was completely different. My tank on Jade (2-tone tourer in signature below) is plumbed as gravity feed, with the belly tank set up to refill the main. I pumped my gas and the wife & I both went to the john at the same time. There was a Harley rider on the other side of the pump. When I went to start it, the choke was all the way engaged - wouldn't start. That's odd. And the high beam was on. A couple minutes down the road and my crotch was soaked with gas. Turns out the fuel pump switch on the fuel control panel was switched on along with all the left handlebar switches. Very funny Harley ahole coward. That could have been serious if it had an ignition source. I've since added a hidden arming switch to the fuel pump control circuit. And a vented gas cap with check valve. And a timer circuit that shuts off the pump a few seconds before it finishes the transfer. And we now have a policy, one of us always stays with the vehicles (including car), never unattended at the gas pumps. I've had 2 other instances of sabotage that I know of in my life, done at the gas pump. When eating in a restaurant - always park where I can see the vehicles. Any miscreant can assume it's being watched by the NRA member owner - it is.