If your vehicle is showing red brake system warning light stays on, here are the most likely causes and the parts you'll need to fix each one.
The parking brake is either partially engaged, or the switch that detects its position is stuck, broken, or misaligned.
As brake pads wear down, the caliper pistons extend further out, causing the fluid level in the master cylinder reservoir to drop and trigger the low fluid sensor.
Many vehicles use electronic sensors that trigger the red brake light when the pads are dangerously thin. This can also happen if a technician forgets to install a new sensor during a recent brake job.
A physical leak in the brake lines, hoses, or calipers is causing a dangerous loss of hydraulic fluid and pressure.
If brake work was recently performed and the system was not bled correctly, trapped air can cause a pressure differential that triggers the warning light.
Internal seals inside the master cylinder have failed, causing a loss of hydraulic pressure in one or both brake circuits without necessarily leaking fluid externally.