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 DiscThrottle Grip and Cables
Free Play Inspection
Refer to the Throttle Control System Inspection in the Periodic
Maintenance chapter.
Free Play Adjustment
Refer to the Throttle Control System Inspection in the Periodic
Maintenance chapter.
Cable Installation
Install the throttle cables in accordance with the Cable,
...
Air Suction Valve Inspection
Remove the air suction valve (see Air Suction Valve Removal).
Visually inspect the reeds [A] for cracks, folds, warps,
heat damage or other damage.
If there is any doubt as to the condition of the reeds, replace
the air suction valve as an assembly.
Check the reed contact areas [B] of ...
Shift Fork Guide Pin/Drum Groove Wear Inspection
Measure the diameter of each shift fork guide pin [A], and
measure the width [B] of each shift drum groove.
If the guide pin on any shift fork is less than the service
limit, the fork must be replaced.
Shift Fork Guide Pin Diameter
Standard: 6.9 7.0 mm (0.272
0.276 in.)
Service Limit: ...