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Which tank is over flowing?
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If the back tank was over flowing while you where running on the front tank then this means you have a bad check valve in the rear tank. The system is designed with a built in check valve in each tank so the return fuel flows back to the tank its being drawn form. If your front pump is returning the return fuel to the rear tank, this means the rear tank check valve in the pump isnt holding. The fix is to replace the rear pump assembly.