[GMGQ]

For my DIY CPR style overflow, I have 2 x 1" U-tubes, and they did lose syphon sometimes (after a power outage or manual shutoff).

The problem was that the intake of the tubes on the inside of the tank were not always 'level' with each other. In a perfect world, both tubes would just kiss the surface of the water, maintaining a bit of suction for when the power came back on. But in reality, one tube may be higher than the other, so when the water level was low, the higher one would end up sucking in air and lose syphon, ending up with an empty U-tube. When the power came back on, that tube was not syphoning anymore, therefore the overflow couldnt keep up with the water being returned to the tank.

Since then I've drilled holes in the tops of both u-tubes and installed an Aqualifter. Now no matter what, when power comes back, it sucks out all the excess air in the tubes to restart the syphon.

Better safe than sorry.