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 DiscTie-Rod Removal
Squeeze the brake lever slowly and hold it with a band
[A].
Remove the front footpeg bracket bolts [A].
Raise the rear wheel off the ground with the jack (see
Rear Shock Absorber Removal).
Special Tools - Jack: 57001-1238
Jack Attachment: 57001-1608
Remove:
Upper ...
Tail/Brake Light (LED) Removal/Installation
Remove:
Upper Seat Cover (see Seat Cover Removal in the
Frame chapter)
Disconnect the tail/brake light connector [A].
Remove:
Tail/Brake Light Mounting Screws [A]
Bolts [B] and Brackets [C]
Bolt [D]
Remove the tail/brake light [A].
Bring down the rear fender rea ...
Release Shaft Installation
Apply grease to the oil seal lips on the upper ridge of the
clutch cover.
Apply engine oil to the needle bearings in the hole of the
clutch cover.
Applymolybdenum disulfide grease to the pusher-holding
portion [A] on the release shaft.
Install the washer [B] and spring [C].
...