1. Disconnect the brake fluid level sensor connector from the master cylinder.
2. Inspect for continuity according to fluid level between the brake fluid level sensor terminals.

If not as indicated in the table, replace the reserve tank.

Brake Switch InspectionAlarm
The Subaru Solterra alarm system combines visual and audible alerts
to immediately notify of any unauthorized intrusion attempts.
The alarm in the Subaru Solterra will activate under the following
conditions when armed:
If any locked door or the rear hatch is opened without using the proper ...
No.7 Bsm Indicator Light Does Not Flash While Under Bsm Indicator Light Flashing
Conditions (With Combination Switch Operation (Turn Signal Switch)) [Blind Spot
Monitoring (Bsm)]
7
BSM indicator light does not flash while under BSM indicator light-flashing
conditions (with combination switch operation (turn signal switch))
Description
The BSM indicator light does not flash or illuminate continuously,
...
Scheduled Maintenance (Mexico)
Follow Schedule 1 if the vehicle is operated mainly where none of the
following conditions
(severe driving conditions) apply.
Repeated short-distance driving
Driving in dusty conditions
Driving with extended use of brakes
Driving in areas where salt or other corrosive materials are used ...