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 DiscDrive 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.
...
Inquiries to Rider
DFI Diagnosis Flow Chart
Each rider reacts to problems in different ways, so it is important to
confirm what kind of symptoms
the rider has encountered.
Try to find out exactly what problem occurred under exactly what
conditions by asking the rider;
knowing this information may h ...
Oxygen Sensor - not activated (Service Code 33, Equipped Models)
Oxygen Sensor Removal/Installation
Refer to the Oxygen Sensor Removal/Installation
(Equipped Models) in the Electrical System chapter.
Oxygen Sensor Inspection
Remove:
Fuel Tank (see Fuel Tank Removal in the Fuel System
(DFI) chapter)
Oxygen Sensor Lead Connector [A]
Connect t ...