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 DiscCylinder (Upper Crankcase) Wear Inspection
Split the crankcase (see Crankcase Splitting).
Remove the pistons (see Piston Removal).
Assemble the cylinder (upper crankcase) [A], the cylinder
head [B] and the cylinder head gasket [C].
Tighten the cylinder head bolts (see Cylinder Head Installation
in the Engine Top End chapter).
...
Spring Plate Free Play Measurement
Insufficient clutch free play will cause the engine braking
effect to be more sudden, resulting in rear wheel hop. On
the other hand, if the free play is excessive, the clutch lever
may feel “spongy” or pulsate when pulled.
Hold an extra drive shaft in a vise and install the following
c ...
Thermostat Inspection
Remove the thermostat (see Thermostat Removal).
Inspect the thermostat valve [A] at room temperature.
If the valve is open, replace the thermostat with a new
one.
To check valve opening temperature, suspend the thermostat
[A] in a container of water and raise the temperature
of ...