Herbie style overflow

[fkshiu]

Hmm, that's interesting. I've got a 90 and a 45 degree elbow on my Herbie main drain and it has never had any difficulty re-starting. A Herbie is completely gravity-based so it's hard to imagine a situation where one would not re-start as long as water was flowing downwards.

What sized pipe are you using? And how much flow do you have going through the overflow? Perhaps with narrow pipes and low flow the valve could be closed enough such that some slime or gunk creates a blockage???