I am experiencing an issue with the ABS at slow speeds, specifically below 5 mph. I get a hard and pulsing pedal and the ABS pump kicks in. this seems to lock out the braking action and the vehicle continues to roll. I have replaced the hubs and speed sensors and also the ABS module with a used box. I have talked to several mechanics and they are stumped. Has anyone had this problem or knows what might be the cause?

My name is Roy and live in Minnesota, 99 Suburban LT 1500 5.7v8