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 DiscAir Switching Valve Removal
NOTICE
Never drop the air switching valve especially on a
hard surface. Such a shock to the air switching
valve can damaged it.
Remove the air cleaner housing (see Air Cleaner Housing
Removal in the Fuel System (DFI) chapter).
Disconnect the connector [A].
Disconnect the hoses [B] from ...
Meter Unit Inspection
Remove the meter unit (see Meter Unit Removal).
[1] Battery (+)
[2] Fuel Reserve Switch
[3] Ground (–)
[4] Unused
[5] Right Turn Signal Indicator Light (LED) (+)
[6] Left Turn Signal Indicator Light (LED) (+)
[7] High Beam Indicator Light (LED) (+)
[8] Neutral Indicator Light (LED) (в ...
Check 3-1: CAN Communication Line Resistance Inspection
Set the hand tester [A] to the ×1 Ω range and connect it
to the terminal [11] and [12] in the meter unit.
Special Tool - Hand Tester: 57001-1394
CAN Communication Line Resistance (at Meter Unit)
Standard: 122 126 Ω
If the tester reading is not specified, replace the meter ...