I recently experienced something that may be similar. The front ABS would start pumping violently when I used the brakes at walking speeds but would work normally at any speed above walking speed. The ABS diagnostics showed a C35 code. After following the test procedures in the service manual, I found that terminals 3 and 4 at the ABS wire connector were not getting power. The 2 terminals on the ECM that supply power to terminals 3 and 4 showed light surface corrosion. I cleaned the terminals on the ECM and tested the voltage at the ABS module wire terminals. Voltage was at spec. after cleaning and the ABS has worked normally since.