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 DiscSeparator 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.
...
Balancer Installation
Position the crankshaft at #2, 3 positions TDC or at #1, 4
positions TDC.
Install the balancer assembly as follows.
Align the punch mark [A] on the weight and the projection
[B] on the crankcase.
Engage the balancer gear and the crankshaft gear.
Insert the balancer shaft until it is bot ...
Battery Installation
Place the battery in the battery case.
Connect the (+) cable to the (+) terminal,
and then connect the (–) cable
to the (–) terminal.
NOTICE
Installing the (–) cable to the (+)
terminal of the battery or the (+)
cable to the (–) terminal of the
battery can seriously damaged
...