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 (Other than US, CA and CAL without KIBS Models)
ZX1000JC
ZX1000JD
...
Cleaning Vehicle before Disassembly
Clean the vehicle thoroughly before disassembly. Dirt or
other foreign materials entering into sealed areas during vehicle
disassembly can cause excessive wear and decrease
performance of the vehicle.
Arrangement and Cleaning of Removed Parts
Disassembled parts are easy to confuse. Arrange ...
Washing Your Motorcycle
Rinse your bike with cold water from
a garden hose to remove any loose
dirt.
Mix a mild neutral detergent (designed
for motorcycles or automobiles)
and water in a bucket. Use
a soft cloth or sponge to wash your
motorcycle. If needed, use a mild degreaser
to remove any oil or greas ...