I believe I know what you did and why they won't work.
You have the feed for the bog coming in the bottom from a bulked fitting thus you have a 4 foot pipe going up to higher than water level and the siphoned isn't breaking. That would be because there's to much draw /head pressure to the supply pipe as the air is trying to break Siphon its being sucked down the pipe .afraid the only way to stop that is with an electric check valve. They are held open with power and when the power goes off. It slams shut.the reason the vent works after dropping a foot the head pressure from inside the bog equals out with the siphone breaker.
You have the feed for the bog coming in the bottom from a bulked fitting thus you have a 4 foot pipe going up to higher than water level and the siphoned isn't breaking. That would be because there's to much draw /head pressure to the supply pipe as the air is trying to break Siphon its being sucked down the pipe .afraid the only way to stop that is with an electric check valve. They are held open with power and when the power goes off. It slams shut.the reason the vent works after dropping a foot the head pressure from inside the bog equals out with the siphone breaker.