Hi folks, we have had a similar-sounding situation and wondered if a cause was ever found. Our pool filter will only fill with air when the pool cleaner is plugged into one of the openings under the skimmer basket. Also this started happening after the pool company serviced the cartridge filter. And we have taken all the steps discussed above. Even bought a new pool cleaner with new hoses, replaced O ring and lubed. Pump shows no air even when the filter is filling with air.