[rohrmann]

As long as the vent connection at the tank is near the top, it could only siphon water out until the water dropped to the vent connection. If, for some reason, the vent was placed closer to the bottom of the tank, then the siphon issue would continue until until the water reached that level. You also would need an additional vent to allow air into the tank to replace the water that is siphoned out, so maybe the connection at the tank is faulty, cracked at the tank where the vent flange is attached.