We have a Forest River Premier 2018 2516g popup. The water pump has worked well, pumping a consistent flow to all faucets. I started to winterize this week and the pump started sputtering and spitting as I tried to draw anti-freeze into the system. I had already cleaned the water filter on the back of the pump and had the siphon tube at the bottom of the anti-freeze bucket. I managed to get anti-freeze through to all the faucets. I have not made any changes to the water system since we bought the popup and I have completed this process without problems each fall. Any ideas why the pump is not performing as expected?