I just replaced my old waterway filter and pump with a new Hayward set and hard plumbed it with PVC. I am having something happen now that also happened with my old filter and pump. At first, the return is going strong but after a few hours the return slows way down. The pressure on the gauge goes down after I backwash it a minute and then rinse and the water from the return goes back to what it was. Any ideas? Thanks.