DIY hydraulic lines are great as long as you've done them previously or can figure out a torque spec for chrome plated brass. It's very easy to over or under-torque the nuts. Too tight and they usually crack although micro cracks can take some time to become 'dangerous (I've done it and I have a good idea what I'm doing) Assembly is straightforward, slide nut onto line, 'flare' stainless outer, fit brass 'cone', insert line end into teflon liner, pull nut back over to fitting and tighten up. The stainless outer bites into the teflon and brass holding everything together. Only needs aboutt 1/4 ~ 1/2 turn after fully seated. The problems occur when the stainless outer gets 'trapped' and makes things difficult to turn so you can't really tell if your applying too much torque to nut. I've tried dipping end of line in brake fluid which seems to help but is messy. You can't use ordinary oil or grease. Wash line down with some alcohol, inspect for any cracks, fit and forget. It really sounds like your mechanic just over-tightened the nut and it failed when crack developed, 6 months of use, 600psi or so caused some flexing of the material and finally a failure. At one time I was a pretty good 'forensic mechanic' figuring out how and why things broke ;D