It's obviously the tank. It's probably a bladder tank and the air pressure in the tank is set way too high. This keeps it from taking in very much water. To properly set the air (precharge) in the tank, first turn off the pump, drain all water pressure from the system by opening faucets. Once the water pressure is at zero, set the air pressure in the tank to two pounds lower than the pressure switches cut in pressure. Tools needed: Tire gauge and possibly an air compressor.