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 DiscClutch Removal
Remove:
Clutch Cover (see Clutch Cover Removal)
Clutch Spring Bolts [A]
Clutch Springs (with Clutch Spring Holders)
Clutch Spring Plate [B]
Pusher [C]
Remove:
Friction Plates (10) [A]
Steel Plates (9) [B]
Spring [C]
Spring Seat [D]
Hold the sub clutch hub [A] ...
Throttle Control System Inspection
Check the throttle grip free play [A].
If the free play is incorrect, adjust the throttle cables.
Throttle Grip Free Play
Standard: 2 3 mm (0.08
0.12 in.)
Check that the throttle grip [B] moves smoothly from full
open to close, and the throttle closes quickly and completely
by the ...
Hub Bearing Installation
Before installing the hub bearings, blow any dirt or foreign
particles out of the hub with compressed air to prevent
contamination of the bearings.
Replace the bearings with new ones.
Install the bearings by using the bearing driver set which
does not contact the bearing inner race.
...