If you've got a 300TD wagon, I'd recommend doing a search for "tank" in posts by chad300tdt and review a few of those threads. He's sketched the wagon's tank and strainer. It's significantly different from the sedan/coupe.

How does the tank screen work on a 123 wagon

Also, how much fuel do you add to full when at 1/4 and having fuel starvation? If it's close to the 3/4 quantity then your gauge should be okay, but if it's more than 3/4 of the tank capacity then your tank is probably okay and your gauge is off and you're running out of fuel. 10-15 liters is close to the reserve limit. The fuel senders can really gunk up, so cleaning one may be a reasonable step in any case.

W123 300D Fuel tank sender question

Oh - and welcome to the forum.