The really weak points are the valves. They degrade much faster than the rest of the mask. The problem is that they get stiffer and no longer work properly. The difference between a good valve and a one that is past itīs prime is barely noticeable especially if you are not an expert. So at least the valves should be replaced on a mask bought from surplus.
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If it isnīt broken, it doesnīt have enough features yet.