Did you know that fittings for hydraulic units are very different from regular fittings for air and instrumentation? The hydraulic fittings have much deeper and thinner threads in order to be more resistant to leakage.
The fitting nut is also longer than that for a CFI fitting. However, the thin threads on the hydraulic fitting make them very sensitive to over tightening. The thin threads will easily deform when tightening the fitting.
If your hydraulic systems leak, make sure you have the right fittings.