Wow. Thanks for that waste.
So I guess if I had a functioning check valve I wouldn't see that milkiness in the clear elbow assembly connecting the pump and the filter?

AFAIK, this EC65 is around twenty years old, I inherited it with the house two years ago. I've never seen DE returning to the pump when the pump's been turned off by the timer (and the Jandy remains open), even for a couple hours. And I've definitely never seen DE in the pool (and the pump/filter's 2 feet above the water level). But as I said, I stop bumping when I start to see it migrating into that elbow (when the Jandy's closed).

About the pressure relief: When I turn off my pump the gauge goes to zero so there wouldn't be any pressure to relieve. Is this normal?