Water pump check valve - fresh tank overflowing

Sometimes they leak due to a tiny bit of dirt or sand or something in the seals, which can get washed out with pump use and have the problem resolve itself.

If it's a more or less chronic problem, rather than replacing the pump (assuming it's otherwise satisfactory and working properly), it's usually a pretty straightforward task to install a separate check valve in the pump suction line, and not too expensive either.