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 Plate, Wear, Damage Inspection
Visually inspect the friction and steel plates for signs of
seizure, overheating (discoloration), or uneven wear.
Measure the thickness of each friction plate [A] at several
points.
If any plates show signs of damage, or if they have worn
past the service limit, replace them with new on ...
Connecting Rod Bend Inspection
Remove the connecting rod big end bearing inserts, and
reinstall the connecting rod big end cap.
Select an arbor [A] of the same diameter as the connecting
rod big end, and insert the arbor through the connecting
rod big end.
Select an arbor of the same diameter as the piston pin
and ...
Exploded View
AL: Tighten the two clamp bolts alternately two times to ensure even
tightening torque.
G: Apply grease.
R: Replacement Parts
G: Apply grease.
L: Apply a non-permanent locking agent.
R: Replacement Parts
Specifications
...