Brake line fittings, from line connectors to bleeder screws, can easily become corroded. Removing them is often a Herculean endeavor and leads to the thread stripping, or rounding of the fitting nut.
Corrosion is the changing of the base metal -- mostly steel -- into another substance like rust, or the white dust of galvanic corrosion. This change results in a chemical welding of metal to metal, as well as a structural weakening of the base material. Corrosion can be external as well as internal to the fitting and on the threads. If no corrosion is apparent but you suspect the fitting try to fit a properly-sized wrench over the nut. If it doesn't fit correctly then there is internal corrosion warping the fitting.
Once the fitting is out carefully examine the threads. They should be clean and sharp on both the male and female portions. Slight crushing of the threads can be rectified by carefully using a re-threading set with the proper size tool. In extreme cases, if the threads are squashed flat or worn down from corrosion, either the fitting or component needs replacing.
Also examine the nut surfaces. They should be flat and the corners should be well defined. If the corners are rounded from the force needed to remove the fitting then replace it. It may be easy to seat and tighten now, but if you need to replace it at a later time, a rounded nut can cause huge headaches that could be resolved with a new fitting.
The rule of thumb is if a fitting works (if it's not leaking), then let it be. These components are temperamental, as well as being a critical part of the brake system. That being said, paying attention to them, as well as the rest of the system, could stave off future problems and make for a safer car.
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