Put a ball valve on the end near the tank so that it won't syphon when you don't want it too. But if you do have a syphon, then you don't need the pump, just the ball valve at the end to open/close when you want some water.
The one thing I don't like about this suggestion though, is that you will have stagnant water just sitting in the lines when not in use. This is the only reason I have the upper ball valve that you first asked me about, as to be able to shut the pipes off closer to the source allowing me to open the lower ones to drain out any water sitting in the lines.