Here we come to one of the most misleading things in the vacuum industry. Most people believe that suction or inches of water lift is the end all be all of vacuum power. But suction by itself is not what cleans a carpet. I have tested every vacuum in my store (over a hundred of them) and not one loses suction pressure with even the fullest of bags. Most people confuse suction with AIRFLOW.
It took me a long time to figure this out and I'm going to tell you how I discovered the difference.
About 7 years ago while remodeling our shop I was using a filter queen cannister vacuum to suck up the debris which consisted mostly of sheet rock chunks and dust. Every 20 minutes I would have to clean the filter, sheet rock dust being as fine as it is I really thought nothing of it. I would know that the filter needed to be changed because when I would put the crevice tool near an object to suck it up it would not be able to do it as rapidly as it did with a clean bag/filter. But if I would stick my hand in front of the hose it would suck all the blood to the center just as it did when it had a new filter hmmmmm. I immediately went to get my gauges, sure enough a filter queen with an entirely too full filter pulled 80 inches of water lift. I ran over to the showroom and got another filter queen that was ready to sell guess what 80 inches of water lift.
This was pretty interesting evidence to someone who sells vacuums for a living. I next grabbed an AIRFLOW meter manufactured by the Baird company which rates airflow on a scale from 1-10, of course ten is best. I put it on the clean ready to sell unit and it pulled a 5 which is standard for a filter queen, but when I put it on the machine with the dirty filters it didn't even register. What this proves is while suction is important Airflow is the key to moving dirt. Suction Almost never dies off even on Central vacuum systems with hundreds of feet of pipe & 90 degree turns and elbows the suction will be the same with a full bag as with a clean bag. Airflow however is directly affected by how FAR AWAY it is from the source and the size of the openings.
Take your vacuum in to the nearest vac shop to test the water lift. Be sure and take two bags with you a dirty and a clean so that you can also see that your vacuum doesn't lose suction after 10 oz of dust, IT LOSES AIRFLOW.
Don't ask for an airflow meter as they probably won't have one.