If you have a spare brake part around and a flared tube end it might help you to just pull the threaded portion back and stick the tube in to the hole, just to get a feel for how they seal. The threaded portion's sole purpose is to apply pressure on the bell edges of the flared tube end, pressing it down on to the cone in the middle of the device, whether it's a brake cylinder or a caliper or the pressure differential switch. There are only two surfaces that need to be smooth, clean, and crack-free - a small ring around the inside of the flare, and a small ring where the inside of the flare contacts the cone. You can take a bright light and a magnifying glass and examine them closely and you'll get a good idea beforehand if they're going to seal or not. All it takes is grain of sand, or a metal shaving, to get trapped there and you can have a leak. Or even just a scratch or a crack.
Once the fluid gets in to the void at the end of the fitting it can easily leak past the fitting end and out of the hole since the fitting end is designed for force not sealing. The fitting is just a clamping mechanism.
Anyway, a bunch of words to say take a close look at the actual fluid sealing surfaces.