My girlfriend drives a 2000 cavalier and when you brake you can hear this dum, dum, dum, dum sound coming from the breaks and the brake pedal like pulses. Would this be because of warped rotors? It doesn't shimmy or make noise when driving normally only when braking. Thanks

btw: car has 62k on it not sure if brakes been replaced.

It sounds like the rotors are warped. The pads really need to be checked as well, particularly since you're not sure when (or if) the pads have been changed. I would check the brakes (or get them checked) as soon as possible.

I would think it is also the rotor. Most OEM rotors on cars are very poor since they aren't made of good metal anymore. To get great rotors you almost have to go after market.