Am following up my first post above. Ignore my post on air in the system, that's not a problem. Since my first post, have done the following and have not solved the problem. Thought the problem might be caused by warped rotors so turned the rotors which did have some warpage. I then installed another new set of AC Delco calipers and within 100 miles both rear units were leaking fluid through the piston seals. Except for the brake lines, which appear in excellent condition, the only things left to replace are the ABS Control Module and the pressure differential switch below it. Your input is appreciated.