Can Studor vents go bad?

Sewer line runs below slab and has about 1/8" per foot slope. Main line has been photographed and is clean. Line has clean out, lav, Studor vent, toilet (Toto). Branch with washer and laundry sink connects below that. Line connects to larger line with proper slope. Everything has been fine for about 18 months. Toilet over flowed during a party. Toilet was plunged, and snaked. Toilet over flowed, then drained very slowly. Main line was snaked and photographed. Toilet over flowed then drained very slowly. Water backed up through clean out. Second Studor vent was installed at laundry sink. Today, toilet would not drain properly, then drained very slowly. Water backed up through clean out and was apparently forced out through wax ring. Any suggestions?
Tom Baker
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Tom Baker wrote:

The Studor vent is an "air-admittance" device. IOW, air can flow *into* the sewer line thru the valve, but air (and water) can NOT flow back *out*.
When the toilet flushes, the big glob of water goes down the pipe, pushing the air in the pipe ahead of it. The Studor valve downstream is of no help in relieving the air. Given that, I have to say that this is not a problem involving that Studor valve.
When the toilet backs up/drains slowly, do you notice bubbling at either of the sinks/washer?
Flush the toilet and at the same time drain lots of water (bucket) down the laundry tub or lav basin. Do they drain OK while the toilet is choking?
Longshot- but the Studor *upstream* of the toilet might be stuck (as you suggested). Under the right conditions, the glob of water from the toilet might create a vacuum upstream (I assume there is no stack vent on this branch). Try this: remove the trap from the lav basin and flush toielt. If it is a vacuum problem, the toilet should now drain freely.
But the fact that the water leaks out the CO seems to go against that theory.
Do some of these experiments and get as much info as possible. My gut feel is that there is some "thing" partially blocking the line somewhere and debris (toilet paper) hangs up there.
Jim
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Jim, Thanks for the advice.
We'll try disconnecting the lav trap. We'll pull the toilet and check the trap. The wax ring is apparently damaged, so we will fix that.
What has me confused is the water leaking out of the clean out up stream of the vent. The clean out * is * at floor level. However, I don't understand how water flows / is pushed out of it. I should think that it would admit air to break any vacuum. TB
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Tom Baker wrote:

Yes, that last part (about the CO leaking) bothers me too; It should only leak if there is a blockage, not a vacuum.
BTW, there is no trap (or, shouldn't be) for the toilet; the toilet bowl has a trap built-in.
Jim
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It appears the problem is obstructions on the City side of the tap. Plumber replaced two year old Studor brand air admittance device. Plumber lifted toilet and found water standing in pipe. Plumber cleared obstruction of roots and paper towel about 10 feet into City system.
This after 25 years of essentially the same system in place. I'm about to write the water folks, but don't expect much in response. Thanks for the insights and support.
TB
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