For everyone searching for a rebuild kit or parts for the 77 - 80 KZ650 rear master cylinders, which are impossible to find, I have good news!
The same piston seal and cup is readily available, and fairly cheap, in a Honda application! I recently had this problem on a 77 KZ650 rear M/C and could not find a source for the 14 mm piston seal and cup. So I raided my Honda spare parts and found that the OEM Honda parts from my 79 CBX 14 mm rear M/C are exactly the same as the OEM KZ650's! There are a few different Honda models that used this rear M/C, too. Certain years of Gold Wing and certain CB750/900 models, mostly 78 - 80.The Honda parts are now installed in the KZ and work great. Piston and other parts are different, so you still need to use the Kawasaki parts there.
-Scott
2-04 R1, 81 CSR1000, 81 LTD1000, 2-83 GPz1100, 3-79CBX, 81 CBX, 3-XS650, 84 Venture, +parts
Quote "speed costs money...how fast do you want to go?" (Which Z movie?)
Universal formula for how many motorcycles one should own = n + 1, where n is how many motorcycles you own right now....
