Fuel leak

I am a new club member and a new owner of an Aircoupe Forney F-1. My airplane has had the header tank removed and now has two 15 gallon wing tanks. My hanger entry has a slight slope and after I top off the fuel tanks (no significant  temperature changes) and push it up the slope a a significant amount of fuel runs out of small holes in the lower bottom of the fuselage and even out of the closed fuel cap of the left wing tank.I am in the middle of my annual inspection and my mechanic and I are trying to solve the problem. So for no leaks found in fuel lines. I am wondering where the fuel overflow valve is in this configuration. The cap vents seem open. Any help would be greatly appreciated! Thanks, Ken Boatright