Help with Cetus Sieve/Pump with air lock

[Mohawk]

I need your advice. I am having trouble keeping the correct flow from the Cetus sieve to the waterfall filter.
I recently removed the sequence pump (5000SEQ20) because it seemed to powerful. Now I installed a new sequence
750 series 4200SEQ12 sequence pump. The problem I am having is when I power up the pump...it will pump for about
3 or 4 minutes then the flow slows way down and the cetus fills up and appears to be air lock problem. What I do know 1. No blockage to the Cetus from the bottom drain and receiving full flow. My questions are 1. In first photo this is the plumbing setup
where I enlarged the exit hole from the Cetus up to 3 inch, converting to 2 inch for pump. Does this appear to be where air could get in before the pump? If yes how would I fix that. Photo 2 and 3 just show the entire setup so you can see the total head etc.
I have lost 2 pond seasons fighting this and have tried several fixes with no luck. Any direction would be much appreciated.

 

[Mmathis]

Has it ever run successfully?

Have you contacted the Cetus company or checked their web site for an FAQ page?

 

[Mohawk]

Has it ever run successfully?

Have you contacted the Cetus company or checked their web site for an FAQ page?

Have not contacted them but did research their manual for correct installation.
It did seem to operate nicely the first summer when I had the primer leaf basket and different plumbing
but had to redo that (when my pit flooded before I had a sump pump during a storm)