But it vents into a closed vessel that only vents at 16 so the system only vents at 16lbs
Think of a water heater over pressure valve
If you cap the open end of the drain line it no longer is an overpressure vent it is a plug.
Another analogy...
If you put a hose on the nipple under the radiator cap and plug the open end of it. That hose will not allow pressure out of the radiator to atmosphere, so it does not matter what the cap pressure is it is plugged and the pressure in the rad will just increase tillt something let's go.
