Most do have cracks. I was just looking at some caps I have and I do not see any way to take them apart with out breaking them. As high as they are from the tank there is not a lot of chance of them leaking anyway. I think if your concerned about it that gluing on a new seal is about your only hope. Or buy a whole new assembly.
When I fill the tank (not over filled) gas does seep out and runs back toward the rear bumper to the point where it has screwed up some very expensive paint on the rear quarter panel.
