(for 1980 300SD)I looked here- take a look at "looping return hose to the injector" here is the site but the said they were temp out of stock
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=29M132RDY2A00P9GQO&year=1980&make=MB&model=300-SD-001&category=E&part=Fuel+Hose
I took this information from another thread. If you need to know how much you will need you may have to get your tape measure and measure it. Or, you might go to Ebay there was a seller there who sold it and would say X amout of hose is enought for a 5 cylinger engine. You can find out what the X is. If you are not in a hurry do some searches and you may find it for $1.79-$2.49 per meter; besure to check the shipping when you buy.
Cloth wrapped rubber FUEL/Diesel or Vacuum hose or line 3.5mm ID X 2.0mm, Made in Germany.
Try:
A 615 476 03 26
or
605-78-05-81
Volkswagen dealers also sell the injector retun hose as the older VW rabbits have similar injector bodies. It may be slightly cheaper to get it from them instead of mercedes. stocked under the VW number N 20 252 5 but Mercedes uses the same hose as well as many others who use Bosch Diesel injection systems.
If you need to fix it right now regular vacuum hose will last a 1 or 2 weeks before it gets eaten up and leaks again. I used a piece of silicone vacuum hose on my Volvo diesel and it was on for a year with out leaks. However, when I took it off the hose was rock hard and had to be cut off and was no longer re-usable.
Also to replace the end plug just cut a short piece of hose and get a slightly oversized very fine threaded screw. Coat the clean screw threads with silicone or anothe sealent and shove it in one end of the hose and let it cure a few hours in a warm area before you install it.