Strainer gets air when I switch from recirc to filter

[BoeTXDN]

CYA: 15 ppm
FC: 3
Ph: 7.4

No clog in the suction line- in fact, it's new. Had a Pentair cartridge filter on it and no air or prime loss.
Running a Hayward Tristar 900 variable speed pump at 2500 RPM 12 hours a day and then cut back at night.

Switched to DE Hayward 6020 two weeks ago. Pool was clean at install. Filter ran fine and pool is crystal clear.

However, lately, within 30 minutes, my pump strainer goes from zero bubbles or air to a drop of almost an inch or more of water.

I've done a backwash and added the correct lower amount to DE. Didn't work.

I removed the grids and washed those off and added correct lbs of DE.

When I go back to circulate, the water drop in the strainer goes away.

Never encountered this with cartridges and it's something that has developed recently.

Thanks to all for your expertise and experience!

[Denniswiseman]

Definately a suction side air leak
Lubricate with silicon grease the pump pre-filter basket "O" ring

[Teapot1]

Only a 1" drop? I wouldn't be overly concerned unless it went further, get in the pool and enjoy.
BTW whats the point of running the pump at 2500rpm for 12 hours then slower at night? Just run it slower all the time if its running 24hrs, slower filtration is better filtration and saves money