Jerry, I haven't changed my felt ring yet. Ive used it on hundreds of jobs at this point. I am careful to keep the head clean after every day and I hose of the felt ring and turn it over regularly. Being that its a used machine I'd replace that though, probably good to have a spare too. If the felt ring isn't creating an air tight seal, it would cause major issues with its clean ability, I'd imagine. The site tube should be replaced too if its cracked (I'm sure you know that).
Another thing to definitely check is the jets on the bottom. Mine are 1.5 flood jets that come standard on the machine. I've heard of guys switching them out which will drastically change the flow the machine is putting out. I've never had an issue with the jets so I continue to run with the stock jets at @ 350 psi. The guy you bought it from may have put larger jets on it.
If I were you, I'd order 3 new jets, a new site tube, and a couple felt rings.
If I run out to my van with the Rotovac connected to the vac hose and the head resting on the carpet, I can hear the vac relief on my truckmount. You shouldn't be able to hear air escaping from anywhere on the Rotovac. My 2" hose fits snug over the site tube.
I do carry a jug of defoamer but its full because I hardly ever need it. I was using Bio Break and Flex Powder with Citrus as my pre sprays, both worked great without foaming issues. Ive recently been using Powermax from Cobb and really like it too. I will say that the Powermax seems to foam a little more but not enough to slow me down at all.
My guess is that there is an air leak somewhere. That could cause your suction and foaming issues. I think it's probably a simple/cheap fix. I had to commit to the tool for a while before I would say I was confident in how I was using it. I had never used a rotary extractor prior to me buying a new Rotovac 360i 2 Summers ago. I also didn't have an experienced tech show me how to use it, I kinda just had to work with it and figure it out. I believe that if you hang in there with it, you may very well change the level of cleaning you are offering and set yourself apart from your competition. That's exactly whats happening with us.
Man I want that thing to work for you guys, I really think it will.