DO NOT PUT A VALVE ON YOUR DRAIN. You have a durso drain, no backup for when that valve they are all telling you to put in plugs up with hair algae. That durso is supposed accept up to 600 gallons per hour. It is noisy as hell at that point. If the return section of you sump is low, you need to add more water. It is a balancing act trying to find out how much to have in your sump to start with. With pump off, after water stops draining pull out the durso pipe to drain the overflow. Fill the sump to the top then start the pump. That level is where you need to run as a maximum level.