Sticky master cylinder
A "sticky" master cylinder may prevent the brake pedal from returning fully to its starting position as well as keep the brake lights on. The master cylinder is a hydraulic mechanism that pushes brake fluid through the brake lines when the brake pedal is pressed. If it sticks, or doesn't return fully, the fluid will not return, keeping the brake engaged and the pedal low.
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Brake Pedal Stays Pressed
If the brake pedal is not returning fully, the master cylinder may not be returning to its starting position. It may get stuck on the hydraulic motion when the pedal is pressed.
Brake Lights Remain On
If the pedal is not returning, the brake light switch will stay engaged. This will keep the circuit completed and the brake lights illuminated.
If, after the brakes are applied, they do not release, the master cylinder may keep the brake fluid in the brake lines to the point where the brakes are activated. This will keep the brakes engaged after the pedal is released.