Master cylinder questions

Another question. I'm having a terrible time finding the right fitting for the front hole on the master cylinder. I bought a kit that came with all the fittings but the one 9/16 it came with will not work. It's like the threads are too short. The one that came off of it looks like the one on the first pic. The 2nd pic is the one I have now. That will gets tight before the line is tight. But I can't find one like the first one in the correct size it's a 9/16×18. I can only find it in a 9/16x20? What gives?

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The special one is a 9/16-20 Inverted Flare Tube Nut for 3/16 Brake Line Tubing. Inline Tube has them. (If you actually need a 9/16-18 you have a very strange master cylinder, and you'll need an Allstar Perf. adapter #50115).