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From what I gather about these pumps, there is a rubber bumper on the front of the pump assembly, in front of the blade. This is supposed to knock the blade back into position if it starts to run backwards. If the bumper is knocked loose, or is compromised in any other way, it won't do what it's supposed to. I would check this, it could be causing your problem. I dont have these pumps, but have heard of this problem, and the solution.
Hope this helps!