Don`t remember everything I read, but think it was mentioned to have a separate fuel source.
I have a 82 240 I have been working on. ($440 $ one in Sig) I made a separate fuel container from a 1 gallon oil bottle.
Drill 2 5/16" holes in the cap, ( might have to file them out a bit for the hose)
Run one hose to the return nipple, ( remove the Cigar Hose) and place in bottle.
Run a second hose to the Lift Pump. ( where the Pre Filter connects)
Be sure bottle if full of Fuel.
This will by pass all the fuel lines from the fuel tank and you can at least get the engine started.
Then if you get it to run, then work back towards the tank.
Here is a good thread on Fuel pressure.
fuel pressure
It was mentioned about the valves in the Lift Pump. Part # 000-090-02-10 for the repair kit.
OM617.951 and .952 Lift pump Repair kit W126.120 300SD W123.133 300D
Rebuild those lift pumps! OM616 OM617
Looks like a price drop. Use to be $10. Now $6.75
VALVE. INJECTION PUMP made by Mercedes Benz. #0000900210
Charlie