This one being sold here is definitely the 3 pin version. The third one is a signal that turns on the pump when grounded. So, the vac switch is only a NC switch that disconnects its two terminals apart once a certain vac level is achieved. See on his rudimentary arrangement that the ground that goes to the pump also goes to the switch and the thinner control wire for the pump goes to the other pin of the switch.
This pump has a relay incorporated basically so other than a fuse, you would connect it straight to the battery, and the relay action is controlled by the third pin once earthed.
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