Did you seal the sump and overflow?
The top of the overflow is mostly sealed.
I have absolutely no surface agitation in the sump, calm as can be. However the air is exiting the reverse durso air hole. It would be pretty hard to reacapture all of this.
I am convinced there is no way to effectively "seal" the sump. This would make it difficult for maintain things and limit other gas exchange. Don't want to sacrifice oxygenation.
So I think the answer is the Herbie method. Create a full syphon and limit how much air is expelled from the system.