Because something's wrong, of course. What it could be... bad switch, break/short in the wiring, bad fuse or relay, or even just bad bulbs.. chances are, your car uses dual filament bulbs for the brake lights, where they'll function as both marker lights (with one filament active) and brake lights (with both filaments energized).
applying the brakes
faulty master brake system
You use them by applying the brakes.
STICKING BRAKE CALIPER
by applying the brakes on full
Is the brake fluid level low in your brake master cylinder in the engine compartment ?
Check the switch at the brake pedal .
it may need some brake fluid
brake lights say lit after ignition has been shut off. lights shut off after hitting the brakes.
My rear window brake light works, but not my tail light brakes on my 1997 Monte Carlo. What is the problem?
Check the fuses for your brakes.
No, if your brakes are flashingyou have crossed circuits.