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 DiscWiring Diagram (US, CA and CAL without KIBS Models)
ZX1000JC
ZX1000JD
...
Sidestand Installation
Replace the sidestand nut [A] with a new one.
Apply grease to the sliding areas [B].
Tighten the sidestand bolt [C] first, and then the sidestand
nut.
Torque - Sidestand Bolt: 44 N·m (4.5 kgf·m, 32 ft·lb)
Sidestand Nut: 29 N·m (3.0 kgf·m, 21 ft·lb)
Install the spring [D] as sh ...
Upper Fairing Assembly Disassembly
Remove:
Upper Fairing Assembly (see Upper Fairing Assembly
Removal)
Headlight (see Headlight Removal/Installation in the
Electrical System chapter)
City Light (LED) (see City Light (LED) Removal/Installation
in the Electrical System chapter)
Remove the screws [A], and separate each ...