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 DiscGear Position Switch Removal
Remove:
Oil Pump (see Oil pump Removal in the Engine Lubrication
System chapter)
Engine Sprocket (see Engine Sprocket Removal in the
Final Drive chapter)
Fuel Tank (see Fuel Tank Removal in the Fuel System
(DFI) chapter)
Disconnect the connector [A].
Remove:
Screws [A]
...
Idle Speed Control Valve Actuator Resistance Inspection
Turn the ignition switch to OFF.
Remove the air cleaner housing (see Air Cleaner Housing
Removal in the Fuel System (DFI) chapter).
Disconnect the idle speed control valve actuator connector
[A].
Connect a digital meter to the idle speed control valve
actuator connector [A]
...
Spark Plug Replacement
Remove the stick coils (see Stick Coil Removal in the
Electrical System chapter).
Remove the spark plugs using the 16 mm (0.63 in.) plug
wrench [A] vertically.
Replace the spark plug with new ones.
Standard Spark Plug
Type: NGK CR9EIA-9
Insert the spark plug vertically into the sp ...