ItalFord, on your leaking calipers after a rebuild, what probably happened is, the pistons were slightly pitted. In ALL Euro calipers I've seen including Cobras, Porsches and other exotics, the pistons are always nickel-plated mild steel rather than stainless. And like plating everywhere, if water gets in the fluid, it pits, then rusts the steel underneath.
There is no fix for working guys; on very rare cars, some people have the pistons stripped, welded and re-plated, and I've made billet pistons of stainless bar stock, but both are unreasonably expensive. Only real choice is to find a used caliper that has pistons in better shape. Or upgrade as you did. It was probably not anything you did wrong and also likely not the rebuild kits' fault.
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