This is the age-old problem with the MC's on these bikes- removing the pesky snapring holding the guts in place.
If you have a Sears store near you, pickup a set of snapring pliers, catalog #47385 It will also help if you have a dental pick on hand. (Bob Wessner has a website detailing his battle with a master cylinder clip at
http://home.att.net/~bobwessner/wsb/html/view.cgi-html2.html--SiteID-756653.html)
First pull out the rubber boot over the exposed end of the piston rod. There will be a small wire clip that holds the boot in place, this should pull right out with a dental pick. The smaller of the two pairs of pliers should just reach in far enough to remove the snapring. If this doesn't work, the other option is to try to pull out one portion of the snapring by hooking the pick into one of the hole in the ring and pulling it free from the groove it's seated in. Be sure to make note of the order in which the parts come out.
After you get everything out, clean the bore thoroughly with denatured alcohol (available at your local hardware store in with the stuff for wood refinishing) and a clean rag. Soak the new piston seal in fresh brake fluid for about 20 minutes before reassembly. The FAQ section of this site has handy tips on bleeding the MC and the rest of the brake lines.
Good luck!
