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 DiscSpecial Tools and Sealant
Hand Tester:
57001-1394
Flywheel Puller Assembly, M38 × 1.5/M35 × 1.5:
57001-1405
Peak Voltage Adapter:
57001-1415
Lead Wire - Peak Voltage Adapter:
57001-1449
Key Registration Unit:
57001-1582
Grip:
57001-1591
Flywheel & Pulley Holder:
57001-1605
Rotor Holder:
...
Brake Light Switch Operation Inspection
Turn the ignition switch to ON.
The brake light (LED) [A] should go on when the brake
lever is applied or after the brake pedal is depressed
about 10 mm (0.39 in.).
If it does not, adjust the brake light switch.
Remove the front footpeg bracket (see Brake Pedal Removal
in the B ...
Ignition System
WARNINGThe ignition system produces extremely
high voltage.
Do not touch the spark plug, ignition coil or ignition
coil lead while the engine is running, or you
could receive a severe electrical shock.
NOTICE
Do not disconnect the battery cables or any other
electrical ...