Might be helpful to post up some pics of the ends of the parts so we can see the threads.. What sort of adapter do you have? What threads on each end? Looked at your vids again, and it looks to me like, as previously noted that the female nut at the end of your hose assembly is not threaded on far enough to seat the pipe that is crimped to the end of the hose. If you remove it, I bet it moves back and forth just like it does when screwed down. If the nut is tight, and the hose is still loose, there is either a bad problem with the threads, or the threads are mismatched - either SAE and metric mismatch, or different pitches. It is quite easy to have mismatched parts. There are numerous flares and the ones you use HAVE to be exact matches. Likewise there are several very close metric and SAE possibilities, but HAVE to have the exact matching threads. All the parts involved need to assemble together smoothly and easily and be obviously made to work with each other. If not there is something wrong. The female nut should smoothly spin at least three full turns (5 to 7 more likely) before suddenly coming to an abrupt halt and all slack removed from the line and caliper. If it gradually gets tight without locking down the hose, there is either thread damage or mismatched threads.