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 DiscOxygen Sensor Heater Power Source Voltage Inspection
NOTE
Be sure the battery is fully charged.
Turn the ignition switch to OFF.
Remove the fuel tank (see Fuel Tank Removal in the Fuel
System (DFI) chapter).
Disconnect the oxygen sensor lead connector and connect
the measuring adapter [A] between these connectors
as shown in the fig ...
Air Switching Valve
Air Switching Valve Operation Test
Refer to the Air Suction System Damage Inspection in the
Periodic Maintenance chapter.
Air Switching Valve Unit Test
Remove the air switching valve (see Air Switching Valve
Removal in the Engine Top End chapter).
Set the hand tester [A] to the × 1 ...
Front Fender Removal
Remove:
Bolts [A] with Washers (Both Sides)
Reflectors (Both Sides, US, CA, CAL and AU Models)
Front Fender [B]
Take the brake hose off the clamp [C] (both sides).
Front Fender Installation
Apply a non-permanent locking agent to the front fender
mounting bolts.
Tighten:
...