Yes and no.

I replaced the slide valve, which was a very easy process (remove 2 screws at the top, pull it out, lube o-rings on new one, put back in, replace 2 screws). I may have been able to just replace the old o-rings, but decided to replace the whole thing anyway.

That stopped the definite hissing/leaking there, but there is still air getting in somewhere else. When I start the pump, I get the bubbles from the returns and/or bleed the air from the top of the filter. When the pump is off, I hear the air bubbles slowly bubbling inside the filter (every 10-15 seconds). I've already opened, lubed the o-ring and reset the top of the filter. I've looked around, but don't see water leaking anywhere but there is definitely something going on.

My next plan was to check the drain at the bottom of the filter (again, and even though it seems dry), the PVC plug on the outlet of the drain pipe and the plug near the heater. I've already looked at them a couple of times but will look again to make sure I'm not missing anything.

Suggestions?