Something that helped me (and I believe was suggested to me here) is to have someone cycle the filter on and off (not too much, a few times) while you are watching the plumbing (including and especially the drain plugs on the pump). The water may spurt out of the leak once the suction is removed (i.e. pump turned off).

This procedure helped me trace a tiny suction leak to the drain plug in my pump (on the suction side, where the strainer basket is). Put some RTV sealant on the drain plug, let it set and no more air bubbles.

-Chris