How can I remove trapped air from my HOB overflow box?

Air keeps collecting at the top of my HOB overflow box. It’s not a U-tube type; it’s an acrylic box with an air vent valve on top.

The problem is that the air gathers in a spot where the vent valve can’t reach. I tried connecting a venturi to suck the air out, but that didn’t work either. The water level keeps fluctuating because of the trapped air.

I also tried running the pump at 100%, but the air still stays trapped in one of the corners.

Does anyone have any suggestions on what I could do?

Thanks