[Sixball]

I just picked up a 95 1/2 Ton Silverado. ABS light is on, and the brakes are spongy. It stops fine, and on the drive home when I stabbed them, the rears momentary locked up, I let off the pedal and the brakes released. The BRAKE light come on once in awhile but only for about 45 seconds or so. Full of fluid. Where should I start looking? Thanks