weeping rear brake lines

In aerospace, we call those copper (or aluminum) washers "soft seats" or "Voishon washers" (a manufacturer). They are even thinner than above and work to high pressures. I know they exist for 1/4" tube sizes, but not sure about 3/16". Aerospace uses a lot of stainless flare fittings, so leaks are not unusual. In a pinch (on my cars, not aircraft!), I have stuck a thin O-ring in a flare fitting, but that has been for power steering or such, not for more critical brake lines.