The twist connection between my Hyperflow pump and associated cap results in significant leakage when a wide mouth bottle is inverted for back flushing. Have you ever done a back flush, using this connection, without experiencing leaks?
This is normal. I think I misspoke when I said that the connection with the cap and pump was air tight. What really happens is when you invert the bottle when backflushing the connection does become air tight due to the water forcing the air up. There will be leakage but it shouldn't be so bad that all of the water leaks out before you're able to backflush properly.
Without using the lid but instead using a typical 20 oz. soda bottle, there will be more leakage than when using the lid, but with practice the user will still be able to backflush.