What is the torque on those damn lines? When I swapped out my radiator, I had to use an 18" Crescent wrench and was only then barely able to break the original connection.
Had to tighten them massively after replacement.
*Edit* So you have confirmed that you have lost the o ring?
I had trouble initially with the upper oil cooler line leaking. Interestingly enough, it did not appear until after a few weeks worth of driving.
I ended up having to remove the radiator shroud upper assembly first, then the lower and torque the line hard...