Do you have anti lock brakes that might be it if not your rotors maybe warped
A leak in your brake line, or very low brake fluid, both of which are extremely dangerous and should be fixed at once.
misadjusted or malfunctioning Idle Speed Control ( ISC ) motor
Your brake light switch is out of adjustment. Don't know what vehicle you have but most brake light switches are located on the brake peddle lever just above the brake peddle pad.
dahil sY0..
There might be something wrong with the brake pads. It is a good idea to get these checked out right away.
There is a small switch just above the break peddle that controls the lights.
Brake pads could be worn or lack of fluid in brake funnel
just above the brake peddle
The Anti-Lock brakes pulsate so the brakes do not lock up (anti) and cause the vehicle to side out.
You may need to check your brake light switch. It is a small switch located on top of your brake peddle (up on the arm of the peddle). This switch allows the brake lights to come on whenever you push the peddle.
Warped rotors. Driving using the left foot to brake causes the rotors to heat up. Quenching the hot rotors by driving thru water will cause them to warp. The warped rotors cause the pedal to pulsate.
Stepping on the gas peddle and letting off the gas peddle.