New filter system now air coming out of jets

[fox_forma]

Well we have a pretty old pool and had an ancient sand filter, prob bout 15yrs old which finally gave out. With this system the jets in the pool bottom pushed out nice and hard and really kept everything flowing. Since we have replaced the filter the pool guys put in a cartridge type and now it just seems all the jets do it push out air. There is some water flow coming out but nothing like before. We had a tech out here and did some sort of pressure test where he put an adapter on our hose and stuck it down the SK and checked for water leaks but cant find any. The pump seems to be running a lot better but where is this air coming from now? The new cartridge filter is a Hayward (cant remember the size) but from what I am told you open the valve on top up until water comes out, shut it and it should stay full of water. Within minutes air is built up and I have to open it again, The pressure is staying at around 10-12psi which they say is normal but where is this air coming from? Tired of dealing with paying people to come out only to leave with the problem still there. Our pool looks like junk every morning cause of this

Any ideas?