There is supposed to be a cap on the top port. If you hook up to the top port(say boat is on a lift and you can't get to the one on the back) then you pinch the hose going to the rear port and the hose going to the feed from the jet pump(pinch when using either port). The flush ports are where the used coolimg water exits the boat. I put ball valves in with a 1/2" inside diameter so I don't need the pincher, simply turn the valve.