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 DiscCam Wear Inspection
Remove the camshafts (see Camshaft Removal).
Measure the height [A] of each cam with a micrometer.
If the cams are worn down past the service limit, replace
the camshaft.
Cam Height
Standard:
Exhaust 35.043 35.157 mm
(1.3796 1.3841 in.)
Intake 38.243 38.357 mm
(1.5056 1.5101 in. ...
Subthrottle Sensor Input Voltage Inspection
NOTE
Be sure the battery is fully charged.
Turn the ignition switch to OFF.
Remove the air cleaner housing (see Air Cleaner Housing
Removal in the Fuel System (DFI) chapter)
Disconnect the subthrottle sensor connector and connect
the harness adapter [A] between these connectors.
Spec ...
Air Switching Valve (Service Code 64)
Air Switching Valve Removal/Installation
Refer to the Air Switching Valve Removal/Installation in
the Engine Top End chapter.
Air Switching Valve Inspection
Refer to the Air Switching Valve Unit Test in the Electrical
System chapter.
If the air switching valve [A] is normal, check ...