More question from the great parts box from Skip Cox, why did they ever stop making the MK5?

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D.A. Aquamaster had a post on this somewhere. From what I remember he attributed the change to the MK-10 because it was easier and less expensive to manufacture than the MK-5.

His post has pretty much the same explanation as Luis's. When they went to the MK10 they wanted the ambient chamber bore and HP piston shaft bore to be more precisely aligned to reduce problems, and the only way they could practically (or economically) do that at the time meant reducing the size of the bore.

Theoretically, a smaller, lighter piston would result in quicker response, but I certainly don't think my MK10s outperform my MK5s in terms of IP recovery and stability.