Help! Fuel filter problem

So I decided to change the fuel filter in my wife's 2002 Ford Explorer, after several years of neglect (yes, I know, I waited too long). It was a pain in the arse to remove the lines on the filter due to rust and corrosion but finally did get them off. a small black plastic sleeve came out with the filter from the smaller diameter supply side line of the filter. I put it back in, looked everything over, hooked it back up, and started it up to check for leaks. ALL DRY. took it off the stands and drove it around for 10 or so minutes, brought it back, checked for leaks again. ALL DRY. So I started cleaning up and in the midst of the tools was a small yellow O-ring that I have NO CLUE where it came from.:scratch: Can anyone give me any ideas as to which of the 3 lines the o ring came from. I assumed the one that the sleeve came out with but its not leaking and I dont know if it goes before or after the sleeve. Sorry for the long post, just trying to get this fixed right.....Thanks.