Called "Tube nuts"
You can get new steel ones at most any auto parts store.
Stainless ones can be had but take some looking (PM me, I probably have some)

But to replace them you'll need to re-flare the brake lines, and if you do that you may as well make new brake lines if your old ones are very old.

Short answer is if the brake lines are OK and the tube nuts are reusable then reuse both. If either are due for replacement then replace both the lines and use new tube nuts.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-