Loosen the Spin-on Fuel Fuel Line between the Fuel Supply/Lift Pump and see if you have Fuel coming out; if not you have a Fuel Supply Problem.
Then there is the Fuel Pressure Relief/Over Flow Valve the controls the pressure of the Fuel Supply/Lift Pump.
Long Fuel Pressure Relief Valve/Overflow Valve Thread
Fuel pressure relief valve adjustment - PeachParts Mercedes ShopForum
There is a Valve Kit for the Fuel Supply/Lift Pump (it used to be around $20) you can get from Mercedes and maybe Pelican. Then you need to scrounge up a 5mm O-ring because that does not come in the kit.
However, before ordering any kit I would remove the big 30 or 32 mm Plug on the Fuel Supply Lift Pump and pull out the Spring and if you can the Piston and inspec them. You need to know if the Spring is broken.
It is also not unknow for the Push Rod to break.
Bunch of random stuff from My notes:
Lift Pump Kit Hunter 617.951-952
OM617.951 and .952 Lift pump Repair kit W126.120 300SD W123.133 300D - PeachParts Mercedes ShopForum
Lift Pump kit post #48
fuel pressure - Page 3 - PeachParts Mercedes ShopForum MORE LIFT PUMP STUFF
FUEL DELIVERY DIY (Hunter)
Fuel Delivery: - PeachParts Mercedes ShopForum FUEL DELIVERY DIY
Lift Pump taken apart
fuel pressure - Page 3 - PeachParts Mercedes ShopForum
Lift Pump, Fuel Supply Pump rebuild kit
Anybody know a good Fuel Pump replacement DIY page? - PeachParts Mercedes ShopForum
Lift Pump, Fuel Supply Pump about sanding the Valves has the small O-ring size
Lift Pump Rebuild... - PeachParts Mercedes ShopForum