Herbie style overflow

[mark]

you realize though depending on how closed the valve is on the primary drain is, the water will back up over the top of the primary.

On my 33g will about 200gph, with everything balanced (surface height steady) the water surface was over 2" above the top of the drain.

Might need also to increase the height of the emergency.

[Captainhemo]

Hmmm, if its actually airlocking, you should be able to drill a small " bleed" hole right above the water line where the drain enters the sump

[Chad]

would that still work if you have the gate valve? should the bleed hole be before or after the gate valve?

[Captainhemo]

Well, this wouldn't be a "normal " placemnet for an air bleed, I'm kinda hoping it might work though....
I'd try putting a small hole right above the water line of the sump (between the water line and the bottom of the gate valve) .