When I first installed my LQ I removed the check valves because the initial flow was slow and it was posted everywhere that if your equip is above water level you don't need'em. This is also what the tech from HASA said.
After the installation I noticed that my pool was fed by not only FC, but also by DE too.
When the pump shuts off, the LQ drained into the inlet line, bringing air into the system. I noticed the pump basket got filled with DE. (I have the Hayward Super Pump with a transparent basket cover.)
I reinstalled the check valve into the "In" line. There was no flow reduction and no more DE running back into my pool!