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 DiscExploded View
25. “1T” marked side faces up.
26. “T2” marked side faces up.
27. Hollow mark faces exhaust side.
G: Apply grease.
L: Apply a non-permanent locking agent.
LG: Apply liquid gasket.
M: Apply molybdenum disulfide grease.
MO: Apply molybdenum disulfide oil solution.
...
Fuse Box Fuse Removal
Remove the left upper inner fairing (see Upper Inner Fairing
Removal in the Frame chapter).
Unlock the hook [A] to take off the fuse box 1 [B].
Unlock the hook [C] to lift up the lid.
Pull the fuses [A] straight out of the fuse box 1 with needle
nose pliers.
Fuse Box 2
...
Main Throttle Sensor Input Voltage Inspection
NOTE
Be sure the battery is fully charged.
Turn the ignition switch to OFF.
Remove the throttle body assy (see Throttle Body Assy
Removal in the Fuel System (DFI) chapter).
Do not disconnect the connectors of the throttle body
assy.
Disconnect the main throttle sensor connector ...