How can I stop the leak in my fuel caps?

I have owned my plane for 4 years. A couple of months ago I was informed that fuel was streaming from the fuel caps during departure. I circled and landed. We applied sealant to the o-rings and, though it still leaked a bit, I continued the flight – long range tanks and a 4 hour flight properly leaned (landed with 15 gallons in reserve).
I started with “full” tanks. After loosing 2 gallons per side, the leaking stopped. I have not had a significant leak since I stopped fully filling my tanks.
the vents for the tanks faces forward. This would tend to pressurize the fuel tanks (on purpose?).
I replaced the post and cap o-rings after taking emery paper to the post seal area and used easy lube on all, but still see some stain if I fill close to full.

Any suggestions?