Air bulding up in filter - PacFab FNS Nautilus

[pineapple1]

Happy Memorial Day! This is our first summer in a home we purchased last Fall from my parents. The home came with an in-ground pool that we are now attempting to open I have to admit, I'm not entirely sure what I'm doing. I've been doing a fair amount of reading and seem to be figuring things out pretty well. However, periodically I've been opening the pressure relief valve on top of the filter and notice that A LOT of air comes out. It will "hiss" for a good 20 seconds before water finally spurts out. Sometimes, water doesn't even come out. It will just stop hissing - but no water.

Assuming something must be clogged, I cracked open the filter tank and cleaned things out pretty good. Not sure what's going on though. Any thoughts on how I might be getting air into the lines?

On an unrelated note, I'm also confused on how much DE to put into my filter. Reading the manual for my filter says to put in about 3lbs of DE. (about 3 coffee cans worth?). I did that, but saw a decent amount of "blowback" into the pool. I also didn't see any DE built up on the filter grids when I opened it up to check the "clog". Maybe I didn't put in the right amount?

Any help anyone can offer would be very appreciated. I'm sort of winging things here and hoping for the best. THANKS!

[pool helper 101]

The air building up in the filter is usually air being sucked into the pump and is either entering the system at the pump or on the suction line.

The first place to check is the pump's lid - make sure the o-ring is sealing well. Examine the valve connections too. Often an air leak will be visible as a slow drip when the pump is off.
Sometimes the skimmer will create a vortex which can draw in air, which then collects in the filter.