Hello Tristan,
Thank you for reaching out to us for support in this matter.
Can you please answer some questions I have so I can better address this situation.
Just to clarify, Your using the supplied siphon break, installed above the water surface of your ATO container correct ??
Next step would be to ensure that the small hole on the siphon break is free of algae, organic bio film and other debris so that it operates normally, cutting off the natural siphon after pump turns off.
Make sure that the end of your top off hose is not under the water level of your sump as this can create issues of vacuum / siphon.
After checking these for me and verifying that all is in working order you can please supply me with a few things below.
Please provide us with the following:
1. Proof of purchase with purchase date (PDF, JPG, GIF or PNG format)
2. Serial number on the back of the pump if one is available
3. Current shipping address
Sincerely Michael