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I believe that motor is controlled by the brake switch under the dash. Can you pull up on the brake pedal and make it stop? Might need to adjust that switch, or maybe you have a faulty switch?
Does the motor stop when you unplug the brake switch?
There should also be a series-wired flow switch in the booster head that completes the circuit when the hydraulic flow is insufficient. Check whether that switch is shorted.
I think the flow switch part number is 18020593. I saw one on ebay right now.