I had a cracked expansion tank in my '85 300TD. I tried using heat from a soldering iron to melt the plastic, then I tried using silicone rubber to no avail.
I continued to have diesel fuel smell in the car from the small amount that leaked from this tank. I checked out the tank in my '79 TD and discovered it was made of metal!
I lived with that for many months, and when I filled up I would only fill to 75% full or less to prevent some fuel from going into the expansion tank (that is what it's for, fuel expands when hot! Gassers use this too, its necessary in all "modern" vehicles.)
I finally spoke with a fellow that was parting out an early wagon and he sold me the metal exp. tank which fit perfectly. Why do you think MB started using plastic in a critical area like this?