Is this a cable operated valve with a knob at the overflow to open and close it? Sometimes, you have to make the operation stiffer. One manufacturer I talked to said to take some nylon tie-wraps and basically tie the cable off to the drain to put a mild kink in it. This will add some friction and allow it to stay up rather than being sucked down. It takes a little trial and error to get it stiff enough without being too stiff. If you don't have an access panel, lots of luck. Adjust it as high as you can get it.