Thanks all for the advice and help, all you guys... Turtles, Chewwong, Jeff Young.
According to the previous owner: "The problem only happens every once in a while when someone flushes too much paper or a tampon or something that is not supposed to be there"
Tenants said they did not do that, but they have 3 men living there, two college boys and their dad. So if one of them gets constipated (sorry) I wonder if that would cause it.
I went to Ferguson Plumbing, and I have to say i have never had such outstanding customer service. everyone was happy, joking around and the place was so clean. We have a plumbing place here called Locke Supply and they always act like you are bothering them.
So Proflo changed the design to make the lid smaller, no longer 6" lid. But they did not roll the part number, you cannot buy the size lid I have anymore but the flappers are the same for my 3" system. Since you can't buy the lid, you also can't buy the access sleeve. Maybe I can get a 6" coupling and a length of 6" pipe with a cap, but then i would have to remove the bottom cap and that may hold the flap in place.
Also of note, the flappers ARE supposed to have an O-ring.
The guy at Ferguson listened to my problem, went and got me a valve, pulled the flapper out of it and told me to try it out. I said how much for just the flapper. He said just your promise that you will tell me how it goes and continue shopping with us.
WOW...
So I will try that out, and spray out both sides of the lines.
Question: If it happens again, what backflow device is recommended? I am thinking of changing to this:
https://www.amazon.com/Rectorseal-9...ocphy=9026279&hvtargid=pla-306983991587&psc=1
or this:
https://www.supplyhouse.com/Sioux-C...rrMlwPvFYdEs05AxREAHWFZNvtXV96g4aAurzEALw_wcB
@chefwong they looked at the flow I had on my phone of my setup, (could not post that here) and agreed with you, the flow looks normal.