Diagnosis
The brake sensor micro-switch is stuck in the "engaged" position, or the lever isn't fully returning to physically release the switch.
How to Fix It
Free up the brake lever and sensor mechanism.
- Locate the small, square plastic sensor attached to the brake lever assembly (usually on the left brake).
- Spray a tiny amount of electrical contact cleaner into the gap where the lever presses the sensor button. Do not use WD-40.
- Work the brake lever vigorously 20-30 times to clean the internal contacts and help the spring return the lever fully.
- Check if the light now turns off when the lever is released.
If That Didn't Work
The sensor itself may be faulty. You can temporarily unplug the sensor connector (follow the thin wire from the sensor back to its plug near the handlebar base) to stop the drain. You'll lose the brake light function until you replace the sensor.
Heads Up
Yes, a stuck-on brake light will drain the battery, albeit slowly. It's also a safety issue for riders behind you. Fix this before your next ride.