It might not be the cylinder but the seal at the back of it. Check for fluid drips at the rear of the cylinder.

Nick at Austins sells the seal from his ebay shop

I changed my master cylinder twice only to find that the carpet, or rather the insulation under it, was pressing against the pipe and breaking the seal. I cut a small bit of the insulation under it and that sorted the problem.

Just a word of warning. The clutch is a pig to bleed.