When a car jumps during breaking this is usually because of a warped rotor, according to Trustmymechanic.com. A warped rotor is a pulsation or jerking that occurs whenever the brakes are applied.
Significance and Cause
A warped rotor occurs because the brake calibres press against the pads and rotors. The contact causes the car to jump or vibrate. Mechanics refer to this as a "shimmy." In addition to jumpy braking, the car may also experience vibration in the steering wheel when travelling at speeds higher than 35mph.
Warped rotors are actually quite common. They are typically caused by wear and tear on the vehicle's brake pads. A warped rotor can also result from overheated brakes, which can occur after a long trip.
Warped rotors can be a safety issue. The rotor affects the vehicle's anti-lock braking system, so it can be dangerous if a person with a warped rotor stops abruptly at a high speed. Also, warped rotors cause more progressive wear and tear to a car overall.
Prevention/Solution for Warped Rotors
A mechanic can repair the rotors. In some cases, the rotors need to be fully replaced. Drivers can prevent a warped rotor by having their brakes inspected on a routine basis. One device that can be used to prevent warped rotors is a shim that places a shield on top of the rotor to prevent wear.