are the end of the tubes going into the pots lower than the end of the tube going into the res? if so then gravity has ya, and i might can help, if your system can accept my recommendation. when the tubes draining the water are lower than the tube feeding the water, then the gravity of the water is pulling the water out of the res. i know you said siphon, so i may be wrong. did you test it with the tubes in the pots? they look dry so I am thinking you didnt, but the moisture may all be in the middle. cuz if they are saturated with water, it should stop them from draining, similar to the concept of those aqua globes, they soil plugs them when its moist enough. so if you can raise your exit tubes so they arent lower than entrance tubes, combined with the saturation of the soil, that should stop the flow.
so i basically told you my idea, then proved my idea wrong, then may have proven it back right. hopefully some of my brainstorming here at least reduces some of yours!!! smoke one for me, im out!!!