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 DiscBrake Pedal Position Adjustment
NOTE
Usually it is not necessary to adjust the pedal position,
but always adjust it when push rod locknut has been
loosened.
Loosen the locknut [A] and turn the push rod with the hex
head [B] to achieve the correct pedal position.
If the length [C] shown is 69 ±1 mm (2.7 ±0.04 in.), th ...
Oil Pressure Measurement
Remove:
Right Lower Fairing (see Lower Fairing Removal in the
Frame chapter)
Oil Passage Plug [A]
Attach the adapter [A] and gauge [B] to the plug hole.
Special Tools - Oil Pressure Gauge, 10 kgf/cm²: 57001-164
Oil Pressure Gauge Adapter, PT3/8: 57001
-1233
...
Separator Operation Test
WARNINGGasoline is extremely flammable and can be
explosive
under certain conditions. Turn the ignition
switch OFF. Do not smoke. Make sure the area is
well-ventilated and free from any source of flame
or sparks; this includes any appliance with a pilot
light.
...