I went through this problem with my truck mount. After I replaced the old float valve with a new one I found it did not fix the water box over flow situation I was having. I realized the Shurflo water pump had too much water pressure and that was what was causing the float valve to not operate properly. I discovered the pump's water pressure could be adjusted by simply turning a screw on the pump. Problem solved. I tried to search the web before hand but sometimes it seems unless you know the answer to your question you will never be able to enter the correct words to search. This video helped: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-aL_3kz7y2zM.html
