What is size of the fittings on our old 1/4 " lines?

Master cylinder nuts "generally" over the years are not necessarily standard SAE flare sizes. The "standard" sizes for the size of tubing is such as the made up ones you buy at the parts stores

Things like some newer distro blocks and prop valves and masters are purposely off sizes so the lines cannot be installed incorrectly. Years ago when you could buy hydraulic operated trailer brake controllers, l you could buy various adapters and tees to fit them. Seems to me GM was really "bad" about off-sized threads and weird thread pitches.

Most Mopar uses larger nuts that are usually still flare nut threads, it's just that it's a thread that would normally be used on a larger line if the nut through-hole was the larger size.

Google "Edelman catalog" or "flare nut specifications" you will find several pdf to download

Here is just one

http://www.stanleymotorcarriage.com/Parts/images/ParkerFittings.pdf