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If you have opened the brake system meaning the master cylinder has run dry or been replaced or the lines disconnected then view the image below and the part I circled.
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The reason for a scan tool is to bleed the ABS system since air can get trapped in it and not manually bleed out via the pedal and helper. The scan tool must command the ABS isolation and accumulator valves to cycle during the bleed to expel trapped out of the HCU.