Brake Problem in 2000 Chevy Cavalier 82,000 miles

When driving my brothers car, when you brake relatively hard (Sometimes not even that hard!) the brakes seem to pulsate and slightly skid. I'm thinking it might be something with the ABS system, or air in brake lines. I know it is kind of a vague description, but what do you guys think?

EDIT:
Also, when braking hard, there is a loud pop noise in the back of the car. Probably warped discs.

Have them ground or replaced.Car Repair Talk's forum.