Pressure loss in Heating System post fitting new Expansion Tank

[Hoppers]

Hi, hope someone can help me with a heating pressure loss problem. My valiant boiler has been loosing pressure over the last few months, resulting in me having to top up the water every week or so. Managed to get a boiler engineer to visit who diagnosed a faulty expansion tank. (Water evident at the valve on top of the EV). I replaced the EV but now the pressure drops to zero within a few hours. I’ve bleed all radiators and there are no visible leaks. Boiler working very well also and the EV was correctly charged when it arrived. If water spurts out of the Auto Air Vent (bottle type) when I depress the valve at the top does that mean it is faulty and would this affect my system pressure. The vent does not leak but perhaps it is not venting the system properly. Grateful if anyone can help. Thanks Alvin

 

[Hoppers]

A AAV has got nothing to depress on the top to my knowledge, I have cleared them out with one of my sewing needles by sticking it down through the nozzle, can you post a photo?,

 

[Hoppers]

I depressed the white plastic insert and water squirted out. I was attempting to lower pressure when I first started to work out why I was losing pressure