What has worked for me... 1) never use a chrome water neck. The iron repops from the parts store are fine 2) make sure the housing and water neck are flat. I usually wind up putting the water neck on the belt sander. 3) Wash everything with clear water. Clean thoroughly with Scotchbrite. Clean with solvent. 3) Dry well. An old hair dryer is handy. 4) Install with a light coat of Permatex #1 5) Torque gently. Too much warps the housing and causes leaks.

The above has always worked. I have not always had good luck with silicone on this joint.