Yes Jamie - if you read what I've said in context - and you've got the jist of it if you read further into your reply - most owners never checked the synthetic dust bags AFTER checking the hose, therefore wasting one or two quarters left in the bags. The filters underneath would be terribly clogged up with dust as well. Clearly if a valve was fitted ,either a blockage in the hose or the bag would indicate there was a problem.
Thats why when you rescue an old clean fan Hoover, you may well find the carbon filters on clean uprights like the TP2/3 are completely clogged up compared to the dust channels/where the main hose travels from, looking a lot cleaner. As a repairer's assistant in the mid 1990s, I used to see a lot of vacs where the motors were actually, still pretty good and owners had chucked away the vacs because they thought they had lost their suction. All that was really required was simple things like changing the dust bag, to cleaning the motor filters behind the bag. Owners just don't bother changing the filters on machines like vacuums - in the same way that they won't defrost their old fashioned freezers until something like an LED or a valve tells them to do so to avoid finding that the doors won't open due to the freezer over-freezing!