I've replaced my pump bowl cover o'ring, cleaned and greased it several times...but still get a small bubble rolling to the top of the cover about every 10 seconds. My cover is old and a little cloudy, so I'm not sure where the bubble is coming from, but it may be from the suction line into the pump. I closed the skimmer and drain valves, and still get the bubble (with pump off). No water leaks with pump running. I'm stumped, what to try?