The drain valve (aka pinch valve) is designed to close around the silicone nozzle to seal the drain from dispensing any liquid until directed.
In order to leave enough room to close around the nozzle, it does not shut completely. When looking down the empty drain you will see about a 2 millimeter gap, or a half-moon shape, when the drain valve is in the closed position.
If you are experiencing any leaks from the drain, see LEVO I: Leaking from the drain