leaking brake valve

I suppose part quality could be a factor.
To get a seal at those flared surfaces isn't always easy. The outside diameter of the tubing is usually quite loose in the inside diameter of the tube nut. To hold a steel tubing straight as possible can help no doubt since it wont confirm as easily as softer tubings. Straight isn't so easy to locate and hold in some applications.
To reputedly loosen a quarter turn and retighten a tad further each time would coax/conform the flared surfaces to mate and seal. Common open end wrenches or cheap line wrenches might round the hex off the tubing nut first though.