Scary!!
Here are my thoughts:
All the dual disc front brake systems on the KZs are very similar. It is a simple system, made up of the Master cylinder, the brake lines, and the two calipers (with the pads to squeeze the discs). There is not a lot to go wrong in the way you experienced, other than poor maintenance of the system. My guess is that the fluid in the system has not been purged and replaced in a long time, and so you have gelled brake fluid, plus moisture in the system causing rust/crud that has jammed your calipers. This is not uncommon in bikes that have been sitting outside for a while, but the extent of your symptoms is rather extreme.
If I were you I would do a complete disassembly of the brake system components (calipers and master cylinder), and if there is any sign of corrosion at all I would at least rebuild each item, and maybe even replace the pistons and calipers if if looks like that is needed. Disassemble, clean, replace parts as needed, lightly hone the cylinders, reinstall parts, bleed system, and test result.
Note that Z-1 Enterprises likely has rebuild kits for your system components.
Also, I would do the same on the rear brake system too. Chances are it has been as poorly maintained as the front.
If you do not feel competent to do this, find a good independent shop to do it for you. Your life depends upon your brakes.
You are very lucky you did not have a serious accident. This should be a lesson to those of us who pick up a new bike to check out all the systems before going for much of a ride. Also, I would question the prior owner about his maintenance of the brake system. Did he represent that it was ready to ride? If so, I would press him to contribute to the fix.
Petaluma and Truckee, CA -- member since Jan. 23, 2003;
PREVIOUS KZs: 1980 KZ750H with 108,000 miles; 1980 KZ750E with 28,000 miles; and KZ750H street/cafe project, all sold a few years back.
This is what I do for fun, not for work. It is art, with a little engineering thrown in.
