The parking brake is designed to supplement the transmission when the car is parked especially on an incline. By engaging the parking brake, the load on the transmission is reduced thereby avoiding unnecessary damage and ensuring the vehicle is firmly parked on an incline. The brake warning light comes on when the parking brake is applied and goes off when it is released. If the brake light remains on when the parking brake is released, then something is definitely wrong. Common reasons the brake will stay on after releasing the parking brake include: low brake fluid, faulty ABS, bad level sensor, loss of hydraulic pressure, misadjusted or failed switch, etc.



A sensor in the brake master cylinder illuminates the brake warning light when the hydraulic fluid (brake oil) drops below a certain point. If the oil gets too low or gets low often even after a refill or top-up, it means there may be a leak in the braking system. A common symptom is the brake light going on or off when turning as the fluid level changes.

If a problem occurs within the ABS, the ABS warning light turns on, sometimes with the BRAKE warning light. If both lights come on together after releasing the handbrake, then the ABS should be checked with a scanning device to determine which part of the ABS is faulty.



The brake fluid level sensor is usually mounted on the side of the master cylinder or on the lid/cap. A bad or failing brake fluid level sensor will trigger the BRAKE warning light.



The brake circuit system is designed in a way that it has two hydraulic lines. In most front-wheel cars, one line works the right-front while the other works the left-rear. On rear-wheel drive cars, one line applies the front brakes and the other applies the rear brakes. A pressure differential switch is usually mounted on the master cylinder or on the brake line exiting the master cylinder. The switch will trigger the BRAKE warning light if there is an uneven fluid pressure in one or more lines when the brakes are applied and could also cause the car to pull to one side during braking.



The parking brake switch triggers the BRAKE warning light on when the parking brake is engaged and shuts the light off when the parking brake is released. A failing switch will make the light stay on even when the parking brake is released. To confirm if the parking brake switch is functioning well, try jiggling the parking brake handle after it is down, if the light turns off or blinks, then the problem is from the switch and should be replaced.



Always make sure the parking brake is completely released if you notice the BRAKE light doesn’t go off by jiggling the handle so that the mechanical part of the parking brake system is completely relaxed and that there is no tension on the cables or levers.

Source:
http://autoclinix.com.ng/2017/11/14/why-your-brake-warning-light-stays-on-after-releasing-your-parking-brake/