Sean, I forgot to mention that it is important to clearly mark the 2 halves before separation as it governs the orientation of the master cylinder/ reservoir. Make a careful note of all parts and relevant locations. Re assembly...... after cleaning and before plating of the halves, do a dry build without the spring, to make sure that you can twist the halves after aligning the cut outs. It will require some filing of the surfaces/edges due to corrosion build up. Lubricate the diaphragm with rubber grease to aid twisting motion. It is a bit tricky because of the thickness of the new diaphragm and having to compress the spring at the same time.
You can get all the parts from " Pieces-Fulvia" in France.
If you require any more advice or photo`s let me know.
Good Luck!
Jaydub.