Its called a vacuum reservoir tank and like was already said one line to the engine the source line, one to to the heater controls the supply line. The tank has a check valve to make sure when engine vacuum drops, as when accelerating, the supply line does not. A leak in the lines, the tank or a bad check valve can make the modes change as they lose vacuum and return to their “at rest” position. The vacuum stays low till it is restored by the engine when engine vacuum rises. The source restored ,the tank and system are returned to normal . The modes now change back to the previous selected position. That being said odds are you have a leak somewhere could also be a bad tank. your going to have to check it out.