Help with overflowing sump system

[Peter Peng]

I have a 40g breeder connected to a baffled 20g sump but I am having a lot of troubles with it overflowing completely after the return pump is shut off which causes a big problem as power outages are frequent.

There are many aspects that causes the overflowing of the sump and ill try my best to break it down

Mainly what happens is when the return pump is turned off, water will flow down into to overflow box and into the sump, but once that stops water will be sucked in from the return pump nozzle that usually pumps out water, but instead it creates a siphon sucking down water into the sump. I cannot move it or unscrew it from my tank which will position it in line with the water, and I also cant unscrew the PVC u turn that has been glued together with some kind of epoxy.
This is my tank :



This is my sump, the left side is the return pump and the right side is where the overflow box leads to


This is my overflow box, works great and when pump is shut off, it only drains water to a certain level and then stops


As you can see in this image, this is what the return pump connects too. #3 labeled is stuck to #2 and since its stuck together, it is impossible to screw it out from the tank as #2 will be blocked by the glass on the left side. #1, the black nozzle can be unscrewed but does not fix the problem.



This is a huge problem for me because if there was a power outage the water would overflow, and any help, solutions to this would be appreciated as I am out of ideas of what to do. If more information is needed or images I am more than happy to provide them

Many thanks