Air in return lines on startup

[jlate]

I was wondering if anyone could tell me why on startup my system is blowing a lot of air back into the pool and then what appears to be either millions of tiny bubbles or DE powder for a few minutes before clearing up. The longer it sits before starting the worse the problem is. It is obvously sucking in air while sitting idle but I can't figure out where. The pump is not loosing prime, so it has to be somewhere on the other side? Any suggestions would be appreciated.