Herbie style overflow

[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) .