Brake fluid is not as easy as,say,trans fluid to seal...but the type of connectors must match. Pipe threads are many times just not tightened enough and rarely need any teflon tape,etc,but old threads,old tubing will give you fits! I know from 50+ years of fighting auto repair... Flared fittings have been around a very long time and are very effective.....if everything is right. The flared fittings depend on the pressure exerted on the back side of the tube flare against the surface of mating flare. ( yeah,I know...old news ) what we ASSUME is enough to force the two surfaces together to complete the seal can be affected by a couple things.....You MUST begin with clean threads,both male and female on either pipe,or flare. Never try to use one type of fitting against the other. Ain’t gonna work too good. Trial fit threaded types before assembly,if possible. Keep a small box of known good fittings of every auto type if working on old cars. Picture the flare nut ,either old or new,not threading into the designated opening easily as a new nut/bolt assembly. Any interference between theads can ,and will,reduce the required pressure to seal tube to flare. I know,roll your eyes and say “this guy is an idiot if he thinks I don’t know this stuff”. We all KNOW it,just is overlooked at times. Never assume that because you torqued a fitting that is has reached the correct sealing pressure. Binding threads have caused hours of frustration. OK class..........there is no test today.