This spring I ran across a couple situations where customers caused hydraulic leaks on their machines. In both cases, the leaks were at the fittings on the ends of hydraulic hoses.
It was obvious that the hoses were replacement hoses, and the customers wanted to blame the guy at the parts counter who had "built" the new hoses. The customers claimed that the hose crimps were faulty. After cleaning the mass of oily mud off where the hoses connected to valves or manifolds, I dusted the areas with talc, operated the machine's hydraulics, and diagnosed that the leak was in the flared fitting. When I went to remove the hoses, both times I had to crank like crazy to break loose the flare fitting's nut. With the nut that tight, I knew from experience to carefully check the flare of the fitting.
Sure enough, the flare had a tiny crack in it. It was hard to see, because when the fitting was off the machine the edges of the crack pressed back together, but when the flare fitting was assembled, the tension spread the crack and caused the leak.
The cure was a new flare fitting, assembled with only "firm" torque on the flare nut. Firmly snugging a flare nut is generally enough to seal it. If it leaks, snug it a little tighter. That's better than tightening it the first time so tight that beads of sweat pop out on your forehead, then discovering that you've cracked the fitting with your overpowering strength. "Tight" is always appropriate, but "crushed" can be expensive.