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 DiscStarter Motor Assembly
Install the positive brush assy [A] and the starter motor
terminal [B] as shown in the figure.
Replace the O-ring [C] with a new one, and install it.
Install:
Brush Springs [A]
Negative Brush Assy [B]
Turn the stopper side [C] to inside.
Tighten:
Torque - Brush Holder S ...
Location of parts
Clutch Lever
Left Handlebar Switches
Rebound Damping Force Adjuster
(Front Fork)
Compression damping force adjuster
(Front Fork)
Meter Instruments
Brake Fluid Reservoir (Front)
Right Handlebar Switches
Front Brake Lever
Throttle Grip
Ignition Switch/Steering Lock
Elect ...
Special Tools and Sealants
Outside Circlip Pliers:
57001-144
Piston Pin Puller Assembly:
57001-910
Piston Ring Compressor Grip:
57001-1095
Piston Ring Compressor Belt, 67
79:
57001-1097
Bearing Driver Set:
57001-1129
Liquid Gasket, TB1216B:
92104-1064
Liquid Gasket, TB1207B:
92104-2068
...