So just recently I noticed the brake hardline to the rear right side is badly rusted right where it bends under tthrough the fenderwell. I need to replace this line but I am unsure of what size fittings I will need. After a little searching here I have found that size 3/16 hard line with bubble flares can be used but I was unable to confirm the correct size fitting that will be required. Does anyone have this information? thanks!

I don't know the size of the fitting but 3/16" tube will work. Recently used NiCop lines for the first time and I'm not going back to anything else, it was incredibly easy to form and is corrosion resistant.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

If the old Fitting is good simply reuse it.

Next best would be to bring the old fitting to the auto parts store and see if they have any.

I don't know if the auto part places have a free rental bubble flare kits. I bought one on eBay and amazon also sells them.

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84 300D, 82 Volvo 244Gl Diesel

Went to every autoparts store in Northern VA this morning no one seems to have the size in stock. The closest I could find was 3/16 4.8mm biffle flare line, but I need a 3/16 4.75 mm line. You think I could get away using using the 4.8mm line?

Update: NAPA came through to me. I got a line from them. For future reference if anyone needs a rear line the size is 3/16 4.7mm

Good info. Thanks.

This site has the line lengths for a regular W123 Sedan

https://www.benzworld.org/threads/need-advise-on-replacement-of-steel-brake-lines-w123-300d-turbo.3056739/

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84 300D, 82 Volvo 244Gl Diesel

Not sure if this is the size you need from old thread but a different year: "No need to fear. I think most U.S. cars switched to bubble-flare ~2000. I dbl-flared some from a JY car for my 60's U.S. cars. Of course, the tube nuts don't interchange. You can find 3/16" tubing sold w/ metric nuts on ebay. From a kit description:
M10 X 1.0 Threads
Connects 3/16" Bubble Flare Lines"
The above is from post number 7:
Brake Line Repair "95 E300D

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84 300D, 82 Volvo 244Gl Diesel

More possible info: https://www.benzworld.org/threads/sourcing-iso-bubble-metric-thread-fittings.1549982/

"Looks like a standard brake line fitting to me - go to the auto parts store and buy the shortest brake for a VW or so, from the same vintage."

"I have bought many of these and they are indeed standard euro / metric brake lines available at most auto parts stores."

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84 300D, 82 Volvo 244Gl Diesel

Any parts store that you choose will have these lines - in both raw and finished form.

Ask for GM/Euro fitting. They come with the correct metric bubble flare nut. If you are making by hand but the little blister pack of bubble flare nuts from the same rack they are handing you the lines from (its always in the back)

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

pssst-
When one of the 83 300cd rear lines cracked open I used a spare brake line for my '59 vw beetle I had on hand. Same size metric fittings.