It may be a vacuum you're seeing in the pump basket, not an air leak. Your pump may be pushing water out faster than the long vacuum hose can replenish. With the vacuum head on, my pressure drops about 6 psi, and sometimes I see that churning at the pump. Yet, when the air bleed is opened when I'm done, no air escapes! If there's no air in my filter, I can't have been sucking air.
If you have an air bleed on the top of your filter, can you get any air out after it's been doing that for a few minutes? If no, then you can't have an air leak, and you'll be chasing a ghost looking for it.