Many people are not aware that there are different types of brake lines, different types of flares (with different unions), different materials that they can be made of, or how to cut, and flare them properly.
The most common brake line size for most BMW’s and particularly true for E30’s is 4.75mm with a M10 x 1.00 flare nut. Often at part stores this will be sold as 3/16 with a M10 x 1.00 flare nut, this is perfectly fine because 3/16 = 4.76mm.
To cut brake lines you should use a small pipe cutter (under 10 bucks), it is critical that you use a small tube cutter because it allows you to cut the tube without pinching it, and generally cuts it straight:
The next thing you need to know is that BMW’s use BUBBLE FLARES, not double flares. This is critical to know, because you can not join a double and a bubble flare together. On the right is a Bubble flare and on the left is a double flare. BMW’s and E30’s use Bubble flares.
The unions are also different for them.
At the parts store you have the option of buying pre-flared lines or rolls of brake line. If you are making your own flares then you will need the appropriate tool. There are separate tools to make the ISO/Bubble flare and the double flare. Some people claim they can make a bubble flare using a double flare kit, but from what I have seen it does not produce a perfect result.
The bubble flare kit is a one step process, you tighten the brake pipe in the holder/jig and then you use the pitman tool with the appropriate 4.75mm insert to flare the brake line. The flat plate with the two holes is used as a depth gauge to get the brake line at the correct height in the jig. The brake line should poke out the same high as the height of that plate.
The double flare kit requires a two step process, that requires you to make an initial flare with the pitman and an adapter, and then the second step with only the pitman. When people attempt to make a bubble flare with a double flare tool they omit the second step, and use the FLAT SIDE of the jig instead of the beveled side you would normally use while making a double flare. However, I always recommend getting the correct tool for appropriate flare job.
Copper/nickle alloy is generally considered the best, do NOT attempt to use regular 3/16 copper piping, the copper pipe made for use in brake lines has nickle in it and is a special alloy to assure it can withstand the normal 3000 psi operating pressure and 17,000 psi burst pressure. It is the easiest to work with, it bends the easiest, is very rust resistant, and looks nice. For that you will pay at least double the price of other brake lines: