Cleaned the cartridge filter today, and inexplicably, I forgot to open the air release valve before taking off the belt and trying to take the top of the filter off. (Of course I didn't forget to turn off electricity to the system.) Finding it difficult to take off the too, I suddenly realized what I did, and I opened the valve. Cleaning went as planned.
Just how bad WAS that, and how lucky am I to be typing this? How big a catastrophe did I avoid?