From the 240 forum, where all the cars are MUCH older than yours, we who work on our brake lines all know that the only flare nut wrenches that work (are more precisely sized not to slip, and are robust enough not to bend) are from a Snap-On dealer (or from their website).
If you're not familiar with the brand, they're the guys that work out of their trucks -- you have to ask around at shops to ask when the Snap-On guys is coming again (they make regular rounds to collect payments, because the guys in the shops all buy on credit. The tools are expensive, but they're worth it, and you only need a couple of sizes, right?

To work on an old 240 (worse than your '97), what I do is first spray with brake cleaner to get rid of the dirt (that would block the PB-Blaster from penetrating); then spray, and respray every few hours with PB-Blaster over the course of a day. By the next morning, you put a pair of Snap-Ons on the lines' nuts, and they will give way easily, and (most important) without having messed up the nuts. This has never failed me over the years, on '80, '83, '84 and '93 240s.

Good luck.