Remove all 6 bottles (empty) and position them so they can be submerged in the water jacket (hoses connected) then run the 6 bottle empty option which is selected with the rotary switch.

Hold each bottle under the water whilst the air pump is running and look for bubbles...This was the only way I disscovered bottle 6 on mine had a damaged thread....Nothing a bit of ptfe plumbers tape didn't sort out.
Also vaseline the grommets.

Cheers Dave