| Another option
I have gone the siphon break hole route, and you have to issues here to contend with. First, the holes will eventually become blocked, usually encrusting algae, unless you turn the system off on a regular basis and "exercise" the system. Worse, under pressure these holes, if not the perfect size, can throw bubbles into the system (if you make the mistake of drilling the hole above the starting water line. So, what to do? A $22.00 check valve will make all these issues go away. All the usual suppliers have them, and they work. | 
