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 DiscExploded View
7. US, CA and CAL Models
8. Immobilizer System Equipped Models
9. ZX1000JD/KD
AD: Apply adhesive.
L: Apply a non-permanent locking agent.
R: Replacement Parts
13. Other than US, CA and CAL Models
CL: Apply cable lubricant.
EO: Apply engine oil.
G: Apply grease.
L: Apply a ...
Tightening Sequence
Generally, when installing a part with several bolts, nuts,
or screws, start them all in their holes and tighten them to
a snug fit. Then tighten them according to the specified sequence
to prevent case warpage or deformation which can
lead to malfunction. Conversely when loosening the bolts,
n ...
Interlock Operation Inspection
Raise the rear wheel off the ground with the stand [A].
1st Check
Start the engine to the following conditions
Condition:
Transmission Gear → 1st Position
Clutch Lever → Release
Sidestand → Down or Up
Turn the ignition switch to ON and push the starter butto ...