Air in system problem

[Fred G]

Hello to all forum members. I'm new to the forum and in need of some help, I'm getting air in the system which shows when the pool switches on and I get large bubbles coming from the return jets. I have had this problem for sometime now and had different pool companies come and see what is causing this problem but non could find it. I have had a pressure test and was told the pipework was fine I have checked and replaced all gaskets{pump lid, Spider gasket and sandfilter} I thought I might have found the problem when I turned off the main drain and that stopped air getting in and it was ok till I vacuumed the pool yesterday I had the skimmer and the vac valve open but left the main drain closed but this morning the air is back in the system. All fittings have been checked for leaks. Has anyone had anything like my problem and been able to sort it?
Thank you for any help given.
Fred.