replacement of old fuel lines is a must. I found this out when I tried to replace my fuel filter and came out with a pinhole leak in a rubber fuel line. It probably would have never leaked even though it was 21 yrs old and hard as rock if I hadn't messed with it. Another thing is that some of the brass washers won't seal properly once they are unseated. That was my problem after I fixed the leaky rubber hose. I had to buy new washers from the dealer because no auto parts store had correct washers to do the job. After new lines and washers and a lot of time trying to get it not to leak, but I just tightened it as much I could and it hasn't leaked since. i hope my story could be of some help here. It seems most of the time I need all the help I can get.

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual