For a 123 chassis you will eventually need to replace these lines:

1. Supply line at the tank.
2. Return line at the tank.
3. Line from the steel supply line to the primary filter
4. Line from the primary filter to the injection pump (actually the lift pump)
5. The injector return lines.
6. Cigar hose from the secondary fuel filter to the retun steel line.
7. The tank vent lines also need to be changed on some occations.

5/16" hose will work for all the line but the injector return lines, you'll need 1/8" for that.

You can get solid viton hose, hose with a viton core, or many people have luck with using the fuel hose you get from Mercedes.

On the new models you may have to replace the delivery valve seals, and some of the o-rings where the steel fuel lines screw together.

Grease Works has viton hose kits, you can also get bulk viton, or bulk viton lined hoses which are much cheaper. Don't use the worm gear clamps they will cut into the hose and you'll have to replace the hose much quicker. You should use band style clamps or reuse the stock MB clamps.

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