Regardless of whether the strainer is plugged or not fuel will not flow out of the fuel line unless the engine end of the line is lower than the tank.

When you disconnect the primary filter remove the tank filler cap and blow into the line. In a second or so you should feel the resistance drop as you blow the fuel back into the tank. Get someone to listen at the tank filler and see if they hear a bubbling noise in the tank. If you don't hear bubbling or feel the pressure drop as you clear the line the chances are that your strainer is plugged.

If your tank has never been out in the life of you car it might be the time to drop the tank. I just finished cleaning the tank on my 1975 155/300d. It is dead easy. You should be able to get it out, cleaned, dried and back in in a couple of hours. This operation does require putting the car up on jack stands. A floor jack makes getting th tank back in a bit easer but it is not impossible to do it on your own. Remember to siphon the fuel out. A full tank in your face is not fun.

When you wash the tank remove the strainer, and the fuel gauge. Use regular dish soap, shosh it around a bit and empty it out. That is about all you can do considering the size of the openings. You will probably need a new fuel gauge gasket before reinstalling the gauge. Order it before you start. MB has them but will probably have to order it.

To dry the tank tie a bit of rag or the like on the end of a stick. Stand the tank on it's corner and let the water settle there. Mop around with the rag and get out the remaining water. If you are anal about cleaning it you can pour in a bottle of alcohol and slosh it around. The alcohol will absorb the water and you can mop it out.

Removing the tank will also gives you a chance to inspect everything under the car in this area. Might also be a good time to replace the rubber fuel jumper hoses that connect the supply and return fuel lines to the tank. My tank had two small vent like hoses on the forward right side. Don't forget to connect them.