Don't rebuild it. Buy a new one. The clutch master cylinder on early Esprits happens to be common across a variety of cars (Land Rover 90 is the one I referenced) and is readily available for next to nothing. Instead of taking a chance with mine and having it leak later on, I ordered a new one. I discarded the external plunger arm and used the same one that I removed from my car. Otherwise, the length isn't the same and it will cause problems when it's time to mount to the pedal box.