Air entering through vent pipe

Will moving the header tank higher and extending the vent to suit cure air being sucked in the system through the vent? I fitted an Airjec which has helped but there is still some air getting in through the vent (tested by holding a jar of water over the vent outlet and the water was sucked in the vent). The tank is about 4 feet above the vent joint which is now in to the Airjec together with the cold fill, feed from boiler and feed to pump which is 9" above the Airjec.

i have questions
1) was central heating running when you observe air being drawn
2) was water drawn from indirect cylinder(hot water) at time?

If the aerjec is fitted as per #4 here Drawing in through the vent is near as dammit impossible. However if its just set up as a de-aerator as per #5 [without the cold feed going into it] then it is possible and you should pipe it up as in #4 to avoid problems

Is the cold feed clear? ie not blocked anywhere.