Sounds like you have a leaking fuel return hose. Those are the cloth covered hoses that jump from injector to injector. Very common for them to fail. The good news is, that is one of the cheapest repairs you'll ever make on a Benz. The braided fuel return hose is usually sold by the meter for only a couple of dollars. I usually have a couple of meters of it sitting on the shelf just in case. Biggest pain about replacing them is that when they get old and leaky, they don't want to come off the injectors. Once you get some hose, try twisting and pulling near the end of the hose. It may break. At that point I usually try cutting/squeezing with a pair of diagonal cutters. Gently, as you don't want to cut into the hose barb on the injector. New hose just gets pushed on the hose barbs. Biggest pain is threading around throttle linkage and by the fuel filter. work slowly and pay careful attention to where the hose was routed. If you can swap out a primer pump, you should be able to do this job. I have a 1983 300D so I am somewhat familar with what you are working on. I usually like to spray off any area where I suspect a leak with carb. cleaner. (do this outside) makes a bit of a mess, but after the cleaner evaporates, leaves a nice clean and dry area. Source of leak becomes pretty easy to see. Now if I could only fix my leaking valve cover gasket!
BobK

Current stable:
'86 190E-172k
'83 300D-358k
'87 560SEL-321k
plus a couple of Chevy trucks