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 DiscSafe riding technique
The points given below are applicable for everyday motorcycle use and
should
be carefully observed for safe and effective vehicle operation.
For safety, eye protection and a
helmet are strongly recommended.
You should be aware of and verify
the applicable safety regulations in
force prior ...
Water Pump Housing Assembly
NOTICE
Do not reuse the mechanical seal and oil seal.
Apply high-temperature grease to the new oil seal lips [A].
Press the new oil seal into the housing with a bearing
driver [B] until it stops at the bottom surface [C] of the
housing.
Special Tool - Bearing Driver Set: 57001-1129
...
External Shift Mechanism Installation
Install the gear positioning lever [A] as shown in the figure.
Install:
Spring [B]
Washer [C]
Tighten:
Torque - Gear Positioning Lever Bolt [D]: 12 N·m (1.2 kgf·m,
106 in·lb)
Assemble:
Ratchet [A]
Pawls [B]
Pins [C]
Springs [D]
Install the shift ratchet ...