The 3" tube is where they insert the pivot axle thingy that the internals pivot around. If it's leaking I wouldn't spend a second on repairing it, get a new. And be sure to get one with the same amount of GPH, because if you occasionally "Do 200", you don't want a lean spot at high rpm because the new pump is inferior to the old. And BTW, don't "Do 200" with this leaky pump, it doesn't live up its as new performance.
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