'81 KZ550..what other model master cylinders fit?

Well, rebuilt the caliper, but the master cylinder is seems to be seized with concrete. I'm guessing the master cylinder set-up is pretty general. Are there other models that will do the job and easy to find? Thanks for any help you can give.

I'm not positive either, but I think most any mc that will clamp on the bars will work as long as they come off a similar sized caliper.

Is the bike a 550-Ltd? Or a 550-A-Standard?

You can use a mc from just about any Japanese bike, including more modern models & Hondas, etc.
The basic criteria: 7/8" bars, mount angle (the Ltd mount is probably angled so that the reservoir will be level), single disk, mirror orifice. If you don't get a vintage Kaw unit, the brake lever may not match the other side unit & the brake lite wiring will have to be spliced.

If the bike is an Ltd, one from any 550-Ltd or KZ650-CSR should work. If the bike is a Standard, then one from any 550-A should work. Either way, there are porbably others as well. The underside of your mc is probably marked 1/2 which is the mc piston bore for single disk models (1/2").

Ebay has a pretty nice on thats brand new for about $50

Yep. usa-motorcycles-inc on ebbaayy sells a variety of new aftermarket units. I kinda recall one KZR member installing one & it woked fine. I don't know if Z1Enterprises.com can get them.

Great, thanks for the help guys. The search is on...