stainless steel brakeline install

They've traditionally been made from rubber tubing, with steel or aluminum connectors crimped onto their ends. Nearly all passenger cars are shipped with rubber brake lines, and they hardly ever fail.

"Stainless-steel" lines are made of Teflon tubing, not rubber. Teflon has a number of advantages over rubber; the chief ones are that it doesn't expand under pressure and it doesn't deteriorate with age. It also resists high temperatures and is chemically inert, so it's compatible with all brake fluids.

However, Teflon is pretty fragile, so it has to be protected from physical damage (chafing, flying rocks, etc.). Although some manufacturers armor their Teflon hoses with Kevlar, most protect the Teflon with an external sheath of braided stainless-steel wire... So that's why armored Teflon hose is usually called "stainless-steel hose". There's no such thing as a stainless-steel brake line that's "not lined with any material"; ALL stainless-steel brake lines are really Teflon lines with a protective stainless-steel-braid cover.
The ends of the hoses have to be securely attached to the brake calipers and the hard lines, so each hose is terminated by threaded hose-ends.

Those hose-end fittings can be attached to the hoses a couple of ways.

The cheap way is to crimp or swage them onto the hoses, like the fittings on rubber hoses. The more-expensive way is to use a two-piece replaceable hose end that captures a portion of the hose between an inner nipple and a concentric outer socket. These hose-ends (often referred to generically as "Aeroquip fittings" because they were invented by the Aeroquip Corporation) are used EVERYWHERE on aircraft and race cars.
Ok... So what's required for a stainless-steel brake line to be DOT-approved?

First, I should point out that there may be lines available that meet all the DOT specs, but are non-approved only because they haven't been submitted to the DOT for approval.

Manufacturers can't legally say that their lines are approved -- even if they KNOW that the lines meet all the DOT specifications -- without actually submittimg them to the DOT.

For that reason, stainless-steel brake lines can fall into three categories:

"DOT approved" - These lines have been submitted to and approved by the US Department of Transportation.

"non-approved" - These lines don't have a DOT approval, either because they don't meet the specs or simply because they haven't been submitted for testing.

"non-conforming" - These lines are non-approved (and non-approvable) because they fail to meet the DOT specs.

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