I just bought a home with a gunite pool; very simple set up with only one valve and a cartridge filter. Upon advice from the pool supply co., I have: replaced cartridge and the o-ring in filter basket. Pressure is at maybe 2 PSI, and there is churning water in the basket and pressure coming out of the returns but apparently lots of air too. Every time I start the pump, I do have to bleed air out of the top of the filter. Suggestions? Thanks!