I tried to change the fuel filter on my B7500 tractor. There is no cut-off valve between the gas tank and the fuel filter as far as I can tell. I think that when you start to unscrew the fuel filter housing there is supposed to be some sort of integral valve that shuts off the gas flow so you can remove the filter. When I started to unscrew the housing a lot of fuel came out, like 2 cups worth, with no sign of stopping. I screwed it back on because I was afraid it would empty the whole gas tank. Could it be possible that the shut-off valve is not working?