O-rings for clear hard fuel lines

[nelbur]

Two or three years ago I replaced all the clear hard fuel lines on my '97 E 300 D with new lines from the dealer. Now the fitting on the outlet of the prefilter is wet with fuel. I'm puzzled as it seems way to early for the o-ring to have given up, and I thought the prefilter was on the suction side of the fuel system and would be more likely to suck air. Perhaps it is, and just bleeds fuel when shut down. I have a few o-rings bought from Mercedes when I bought the lines, but I wonder if there are better ones to use on these fuel lines. I'm thinking that the ultra low sulfur fuel might have something in it that the o-rings don't like. I saw that A u t o h a u sAZ had some for $3 each, so they must be quite special. Does anyone have any information on these fuel line o-rings and recommendations as to what one should use with today's fuel?