Herbie style overflow

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On you main drain have you tried removing the 90° or having the opening facing the top?

If you have and you finding it sucking air, option is to redrill on the side, just lower down right towards the bottom (immediately below existing main), then place the upturned 90°. The original main drain then could be either plumbed back as another emergency or blanked off.

Can also give up the idea of the Herbie, and run a external Durso. On the main, leave the 90° pointing down, on the outside of the tank have a tee. Cap the top, then drill out for the air (starting will a small bit, or tap for a air valve), the downward leg to the sump.