Fuel Tank Issue

My left fuel tank for whatever reason is building up pressure in it whenever the airplane sits. It vents fuel back through the vent line when this happens. The fuel level is below the vent line level in the tank filler neck. I figured it was probably a partially clogged vent but I have checked the vents, blown them out both directions with no issues. I can't figure out what the heck is going on. I never have problems with the tank feeding or leaking in flight. I didn't build the airplane but I am also wondering if maybe the fuel vent line inside the tank is possibly loose on the fitting where it exits the tank? I recently rebuilt the tank caps with new parts from Vans and I have no leaks anywhere. The right tank is ok, changed the cap from left to right, doesn't help. Anybody have any ideas?

thanks

Rivets

Steve is that pressure build-up causing some rivets to weep?

Where the vent line inside the tank is attached to bulkhead fitting at the rib may have a leak, lets fuel into the line when it should be only vapor.

No leaks or weeps. I'm afraid of the vent line inside the tank has a leak or the vent connection and that means taking it apart. I could probably always burn the fuel down below half and leaving it that way until refueling. It vents ok into the tank which would obviously be a problem if it wasn't. I leave the tank cap unlocked where it will vent, airplane is always inside. I'm going to try lowering the fuel and see what happens and nurse it until after flying season.