So as I was checking to see if back washing was necessary I noticed a little air in the pump basket. So I started to play with the valves. It seems when i turn off the skimmers the bottom drain has almost no air bubbles in the pump basket box window. Whren I shut off the main drain valve and leave just the skimmers on, there is a noticeable amount of small air bubbles. When both valves are open the air bubbles pretty much disappear. I assume this means a problem somewhere. The filter runs great, the pressure is constant at 14psi and i can't notice any air being sucked in. Any suggestions?