Disc brake master cylinder rebuild
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
Thanks to Charlie Mullendore of Antietam Classic Cycle for providing this information on the old Yahoo! Loopframe_Guzzi news group (which has now moved to Groups.io).
To remove the piston/seal assembly of your master cylinder, you'll need to insert a rod of the largest diameter that will fit into the outlet hole and then drive the piston assembly out. The retaining ring is sort of tapered with an edge that catches and holds the whole thing together. I use a 14 mm Craftsman socket and my bench vise to press the new ring and piston assembly into place.
I made ahonefor the master cylinder bore out of a length of 3⁄8 inch aluminum rod. I drilled and tapped a hole into the end of it, cut two discs of Scotchbrite approximately 5⁄8 inch in diameter and then affixed them to the end of the rod with a screw and washer. Chuck the rod up into a drill and have at it - works very well.