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 DiscExploded View
5. Other than US, CA, and CAL Models
6. The big diameter side of the dampers faces upward.
G: Apply grease.
L: Apply a non-permanent locking agent.
R: Replacement Parts
S: Follow the specified tightening sequence.
SS: Apply silicone sealant.
11. Immobilizer Ampli ...
Special Tools and Sealant
Oil Pressure Gauge, 5 kgf/cm²:
57001-125
Hand Tester:
57001-1394
Needle Adapter Set:
57001-1457
Extension Tube:
57001-1578
Fuel Pressure Gauge Adapter:
57001-1593
Fuel Hose:
57001-1607
Measuring Adapter:
57001-1700
Liquid Gasket, TB1211:
56019-120
...
Battery Installation
Visually inspect the surface of the battery container.
If any signs of cracking or electrolyte leakage from the
sides of the battery.
Put the battery into the rear fender
Install the band [A].
Connect the positive (+) cable [B] (red cap) to the positive
(+) terminal first, and then ...