When starting my pool pump the presure starts off at 18 PSI, all equipment working fine, but after 10 minutes the pressure drops to 8 PSI and the pump motor starts making twice as much noise like its straining. I notice the return water going into the pool amounts to half the flow it first started with. I do see a very few amount of bubbles coming out in the return water lines. Do I need to replace the rubber seal in the pump cover over the skimmer, or replace the seal in the cartridge filter, or replace the air release valve on top of the filter, or do I have to go as far and replace the seals in the pump motor. I am troubled as to where to search for the air leaks, can anyone help!