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 DiscEngine No. ZXT00JE003022
1. Heat Insulation Rubber Plate
2. For Air Switching Valve Hoses (Place the air switching valve above the
rubber plate.)
3. For Air Suction Valve Cover
4. For Throttle Body Assy Holder #2 and #3
5. For Throttle Body Assy Holder #1
6. For Water Temperature Sensor
7. Insert the tab into th ...
Air Suction Valve Installation
Install the air suction valve so that opening [A] of the reed
faces downward.
Apply a non-permanent locking agent to the threads of
the air suction valve cover bolts [A], and tighten them.
Torque - Air Suction Valve Cover Bolts: 9.8 N·m (1.0 kgf·m,
87 in·lb)
...
Rear Caliper Holder Shaft Wear
The caliper body must slide smoothly on the caliper holder
shafts [A]. If the body does not slide smoothly, one pad will
wear more than the other, pad wear will increase, and constant
drag on the disc will raise brake and brake fluid temperature.
Check to see that the caliper holder shafts ar ...