I have rust in a place many 300D owners may have also but do not suspect. It coincides with the two metal channels that runs each of the passenger and driver seats. Water collects along the edge and eats away the metal underneath the channels underneath the seats.
This and the rear passenger floorboards are the two main rust spots on the W123 300D series.
The easiest way to weld is to patch from underneath here. But there are three fuel lines in the way held on by brackets. The lazy solution was to cut the sections out and replace them with rubber hose later, but i wouldnt do that. I am going to remove them.
When i disconnect them, particularly the supply line, will fuel come gushing out of my tank? I have about 8 gallons in there now. Has anyone done this before without being doused with diesel.

Yes, fuel would come gushing out. Is it drivable? If so, drive/run it out of fuel.

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1989 300E 144K

Remove the tank rubber hose/ screen and drain the fuel this way. Then blow out the line with compresed air, About a 1/2 cup will come out

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car