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 DiscCamshaft Removal
Remove:
Cylinder Head Cover (see Cylinder Head Cover Removal)
Crankshaft Sensor Cover (see Crankshaft Sensor Removal
in the Electrical System chapter)
Using a wrench on the timing rotor bolt [A], turn the crankshaft
clockwise until the line [B] (TDC mark for #1,4 pistons)
on the timin ...
Drive Chain Wear Inspection
Remove the chain cover (see Drive Chain Removal/Installation
in the Final Drive chapter).
Rotate the rear wheel to inspect the drive chain for damaged
rollers, and loose pins and links
If there is any irregularity, replace the drive chain.
Lubricate the drive chain if it appears dry.
...
Subthrottle Sensor Input Voltage Inspection
NOTE
Be sure the battery is fully charged.
Turn the ignition switch to OFF.
Remove the air cleaner housing (see Air Cleaner Housing
Removal in the Fuel System (DFI) chapter)
Disconnect the subthrottle sensor connector and connect
the harness adapter [A] between these connectors.
Spec ...