Modeling a closed tank

Is there any way to model a closed-tank in WaterCAD? I am modeling a canned pump station. The can (cylindrical shape) have air valves that can operate either under atmospheric pressure or under pressure depending on the source of water to the can. I have two tanks that can feed each can by gravity: chlorine contact basin or above-ground storage tank. When the flow comes from the chlorine contact basin, the cans would never be full (will never be pressurized) and the air valve would open to bring in air to the can to atmospheric condition (since a vacuum would be created when the water level in the can drops). However, when the above-ground storage tank feeds the can by gravity, due to the higher HGL of the storage tank, the cans will fill up and become pressurized. in this case, the air valve would let air out of the can and the can would operate as a sealed tank. I am not sure if I could do both these functions with one model set-up. I have used a tank element when I know the cans would be under atmospheric conditions (fed from chlorine contact basin) and have used pressure junctions for the cans when they are operating under pressure. I was hoping to be able to do both in the same model.