A clutch master cylinder has only 4 parts: an iron body, an aluminum piston, a steel return spring and the RUBBER seal. Replace your cheap neoprene seal with a Viton part and the mostly metal assembly will not leak. My Girling all-aluminum master cylinder was installed in our Pantera in 1981. No problems yet, 'cause it uses a Viton flurocarbon seal (that I added), and I change fluid regularly.
Unless of course the constant use of old brake fluid with significant water in it has rust-pitted the iron bore. The pits will tear any seal and leak. If no yearly change-of-fluid persists, a stainless steel body and an anodized piston will be required. Rebuilders add these at rather high cost. Changing fluid is cheaper....