I've used several, including SnapOn Koolkare Xtreme and a couple of Robinaire units. You'll want to check the condition of the vacuum pump oil, specifically to see if it dark and cloudy or clear and fresh. Generally gives an indication of whether or not the unit was serviced, as it is often overlooked or ignored. It should have a meter indicating how long the present recycling filter has been in use. Most are only rated for a certain number of hours of runtime before they need changing. The SnapOn filter is something between 50 and 90 dollars, depending on where you get it from. Look for cracks in the hoses, loose fittings, etc. Pull a vacuum on the hoses and pause once it hits 29 to 30 inHg. Let it sit for ten or fifteen minutes and see if it leaks down. Also note how quickly it pulls down, faster being better. Check the pressure test certification date on the recovery tank to make sure it isn't expired or near expiration. A unit with a micron gauge will be better, and probably cost more. It would be great if you could check the gauge against a a known accurate gauge, but that may not be possible. I prefer units that have the ability to inject oil, but it isn't completely necessary.
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