If any is worn down to the service limit, replace the brush assy.
Starter Motor Brush Length [B] Standard: 12.05 mm (0.4744 in.) Service Limit: 6.55 mm (0.258 in.)

Starter Motor Assembly
Commutator Cleaning and InspectionPrecautions
There are a number of important precautions that are
musts when servicing electrical systems. Learn and observe
all the rules below.
Do not reverse the battery cable connections. This will
burn out the diodes on the electrical parts.
Always check battery condition before condemning other
...
Front Wheel Rotation Sensor Wiring Inspection (Service Code b 43)
Disconnect the front wheel rotation sensor lead connector
[A] (see Front Wheel Rotation Sensor Removal in the
Brakes chapter).
Disconnect the KIBS hydraulic unit lead connector (see
KIBS Hydraulic Unit Removal in the Brakes chapter).
Check the wiring continuity of the W lead and ...
Turn Signal Relay Inspection
Remove the battery (see Battery Removal).
Take the turn signal relay [A] off the rear fender.
Disconnect the connector [B].
Connect one 12 V battery and turn signal lights as indicated
in the figure, and count how many times the lights
blink for one minute.
Turn Signal Relay ...