98 abs problem

the brake pedal feels like it gets stuck when pressed lightly and then gives in to pulsing like the abs is engaged followed by a sound that is like an electical pump running. what gives??????

Is it intermittent or continuous, the humming?

intermittent, but once on it stays on for about 10-15 seconds. this is repeated every so often when brake is applied. i checked for leaks with the brakes applied and did not notice any. noticed that the master cylinder might have leaked around the booster at one time or another (i am the second owner) so i topped the reservoir and am monitoring.

Describe the conditions that this happens when driving.