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 Cover Installation
Using a cleaning fluid, clean off any oil or dirt that may
be on the silicone sealant coating area. Dry them with a
clean cloth.
Apply silicone sealant to the area [A] where the mating
surface of the crankcase touches the clutch cover gasket.
Sealant - Liquid Gasket, TB1211F: 92104-000 ...
Crankshaft Sensor Peak Voltage Inspection
Refer to the Crankshaft Sensor Peak Voltage Inspection
in the Electrical System chapter.
If the reading is within the standard, remove the ECU and
check the wiring for continuity between harness connectors.
Special Tool - Hand Tester: 57001-1394
Disconnect the ECU and sensor connector ...
Rear Frame Front Removal
Remove:
Rear Frame Rear (see Rear Frame Rear Removal)
Rear Fender Front (see Rear Fender Front Removal)
Rear Footpeg Bracket Bolts [A]
Rear Footpeg Bracket [B]
Rear Frame Front Bolts [C]
Left Rear Frame Front [D]
Remove:
Rear Footpeg Bracket Bolts [A]
Rear Footpeg Bracke ...