This sounds eerily similar to an issue I just had over the last few days. I have an 84 500SEL, so we may have very similar engines. This is going to sound stupid, but is there gas in the tank? That was ultimately, I think, the main culprit in my situation. I will thank you to not judge me (broken gas gauge)
In my defense, I also managed to trace the fuel flow from the tank, through the FD, to the WUR, but not back to the FD. In addition to adding gas to my gasoline powered engine, I also disassembled the WUR and cleaned it really well. There was, I think, some build up inside the WUR that was restricting the flow back to the FD as I had a hard time blowing through the "inlet" hole of the WUR. After cleaning and reassembly, and gas in the tank, she fired right up.
I would suggest adding 5 or 10 gallons of fresh gas and try disconnecting the fuel line from the FD to the WUR and see if the fuel is getting through the FD. Then hook it back up, and disconnect the inlet fuel line (the big fitting. Mine was 14mm) on the WUR and see if fuel is getting there. Then hook it back up and disconnect the outlet fuel line (the small fitting. Mine was 12mm) on the WUR and see if fuel is getting through the WUR. Then hook it back up, follow that return line back to the FD, disconnect it and see if fuel is getting back to the FD. Then hook it back up and disconnect the main fuel return line from the FD back to the tank and see if fuel is getting all the way around the "circuit". While that main return line is disconnected, it might not be a bad idea to blow some air from your compressor back through the return line into the tank to see if it is restricted. You should hear bubbling in the tank. Remember, you are working with gasoline, so safety first. Also, it's not necessary to crank too long, just long enough for your helper to verify gas is coming out of whatever fitting you have disconnected into the towel that is being held over the open fuel line to avoid spills.
I really don't know what I am doing, but this is what I did to track down my problem. I hope it works for you too.