Decided that I had nothing to lose, so disassembled the mc28 master cylinder, swapped the guts into the mc21 master cylinder, and it works great. The 21 MC had worked the last time I rode the bike, about 3-4 years ago, but internals had frozen-up and dried out in spots since then.
First time I've torn into a master cylinder, and glad I did.
Thanks to all for their help, advice, and links.
Jeff