[RightSaidFred]

Had this with mine. It was a horryfiying discovery what the problem was.
The banjos on the ends of the lines (goodrich) were anodised alloy ones. The alloy corrodes electrolytically with the steel of the line and the olive inside. The corrosion gets so bad, it swells inside the banjo and cracks the outer casing. This was then drawing in air for the 'fizzing' but not enough to actually leak fluid out. New lines = problem solved.
These alloy banjos are banned now by the way!