No, the water will not drain back into the pressure tank. During normal operation of a well pump system there is a pressure control switch, which reads water pressure within your pipe line. The switch controls the flow of electricity to your pump when you are using water from your system. When you stop using water, your pressure tank will fill with water until an upper pressure setting is reached and at that moment the pressure switch will interrupt the flow of electricity to the pump and the pump will stop pumping water. A valve, just above your pump will close and hold the water pressure in your system. When you use enough water, the cycle will start over.