OK beer guy...I know what Rolling Rock is if that's what you're referring to. Latrobe, PA.

The only kind of a leak that would significantly affect pressure would be an air leak on the suction side and that would introduce air in the system, bubbles out the returns and a drop in pressure. I know someone has the answer to the question of how much DE you put back, not me. I use sand. What are your pressures? Starting after backwash and how much does it rise. May be a clue to what's happening. Best I can do for now. We'll keep this near the top of the list so it gets views.
Al