i`m in southern mexico and have sand fliter and concrete pool 15 meters x5 meters x1.5 deep.about 29,000 gals.it has 8 returns 1 at each end and 3 on each side.i`m getting air at 1 of the end returns as well as the the first one on each side.i tried opening all the connections and checked all the o rings even replaced the o rings on the filter head and waste basket.i can`t find silicone grease here to seal the orings.so i`m just wondering what it could be.the further away from the filter the less air in the returns.thanks for any help