You can leave the valve open, as long as no water is getting out. I know this because i left my drain valve open one time after maintainence on the filter. only the rubber tip for the valve was keeping the water in. the filter ran the same. i noticed it the next day. this could have been a huge disaster! good thing theres a plastic nut to hold the rubber stop on. yours should be fine also. you could try it out for 24hrs first and see how it goes.