If you visually inspect the o-ring and its not torn, I would suspect that it's not the problem. Having problems with those o-rings is pretty rare, I have never heard of them giving problems.
Have you checked the sending units for rust? That is a more common problem. You may find some dampness on the lines wherever the leak is.
Have you checked the sending units for rust? That is a more common problem. You may find some dampness on the lines wherever the leak is.