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 DiscAir Suction Valve Installation
Install the air suction valve so that opening [A] of the reed
faces downward.
Apply a non-permanent locking agent to the threads of
the air suction valve cover bolts [A], and tighten them.
Torque - Air Suction Valve Cover Bolts: 9.8 N·m (1.0 kgf·m,
87 in·lb)
...
Crankshaft Sensor Inspection
Disconnect the crankshaft sensor lead connector (see
Crankshaft Sensor Removal).
Set the hand tester [A] to the × 10 Ω range and connect it
to the crankshaft sensor lead connector [B].
Special Tool - Hand Tester: 57001-1394
Crankshaft Sensor Resistance
Connections:
Meter (+ ...
Special Tools
Bearing Puller:
57001-135
Head Pipe Outer Race Press Shaft:
57001-1075
Steering Stem Nut Wrench:
57001-1100
Bearing Driver Set:
57001-1129
Steering Stem Bearing Driver, 42.5:
57001-1344
Steering Stem Bearing Driver Adapter,
41.5:
57001-1345
Head Pipe Outer Race Driver, 55:
57001- ...