Strange phenomenon. I had a 2# check valve in my 2" line after filter, "to" the pool. I just removed it because I thought it was interfering with flow on low motor speed. Pump is 12" above water level so I saw no need for it.
Filter pressure on low with check valve: 8 psi
Filter pressure on low without check valve: 12 psi
I would have though it would drop to 4-6. Does this make sense? Possibly more water is moving without the check valve, so filter is seeing more pressure? It is a pressure valve on top of the filter (not a vacuum valve), right?