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 DiscCharging Condition Inspection
Battery charging condition can be checked by measuring
battery terminal voltage with a digital voltmeter [A].
Remove the seat (see Seat Removal in the Frame chapter).
Disconnect the battery terminals.
NOTICE
Be sure to disconnect the negative (–) cable first.
Measure the battery ...
Coupling Bearing Inspection
Since the coupling bearing is made to extremely close
tolerances, the clearance can not normally be measured.
NOTE
It is not necessary to remove the coupling bearing for
inspection. If the bearing is removed, it will need to be
replaced with a new one.
Turn the bearing in the coupling b ...
Thermostat Installation
Replace the O-rings [A] with new ones, and apply grease
to them.
Assemble:
O-rings
Thermostat Housing [B]
Thermostat [C]
Thermostat Cap [D]
Install the thermostat [A] in the housing so that the air
bleeder hole [B] is on top.
Tighten:
Torque - Thermostat Cap Bolts: 5.9 N ...