If a master cylinder or piston shows any damage, replace them.

If a cup is worn, damaged softened (rotted), or swollen, the piston assembly should be replaced to renew the cups.
If fluid leakage is noted at the brake lever, the piston assembly should be replaced to renew the cups.
If they are damaged, replace them.
If the springs are damaged, replace them.
If the relief port becomes plugged, the brake pads will drag on the disc. Blow the ports clean with compressed air.
Front Master Cylinder [I] Rear Master Cylinder [J]

Rear Master Cylinder Installation
Brake DiscIntake Air Temperature Sensor Resistance Inspection
Remove the intake air temperature sensor (see Intake Air
Temperature Sensor Removal).
Suspend the sensor [A] in a container of machine oil so
that the heat-sensitive portion is submerged.
Suspend a thermometer [B] with the heat-sensitive portion
[C] located in almost the same depth with ...
Wiring Diagram (US, CA and CAL without KIBS Models)
ZX1000JC
ZX1000JD
...
Bolts, Nuts and Fasteners Tightness Inspection
Check the tightness of the bolts and nuts listed here. Also,
check to see that each cotter pin is in place and in good
condition.
NOTE
For the engine fasteners, check the tightness of them
when the engine is cold (at room temperature).
If there are loose fasteners, retighten them to ...