The following conditions will activate the BRAKE warning light: The parking brake is applied. The light should be on whenever the parking brake is applied and the ignition switch is on. Check THIS The fluid level is low. A low fluid level in the master cylinder will turn the BRAKE light on.
Check fluid level first. If that is not the issue, make sure the parking brake switch that turns on the light is working properly and set in the correct position. The third reason it could be on is a failure at one of the lines or wheels. This would typically also show by seeing some fluid residue around a wheel (after taking the tire off) and the reservoir would show as low. If you had to add fluid to bring it to full when checking, watch it for a few days and see if the level goes down indicating a leak (and if low enough, your light should come back on).
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