No Vent system in home sewer system

[bicycler]

I have a house that used to be on a septic system, converted to city sewer. The old septic tank still receives waste from the basement drains and toilet. There is no kind of vent anywhere on the sewer lines. Most times the toilet and all drains work fine with the exception of an occasional gurgle. But when it rains really hard the toilet bubbles VIOLENTLY and then sucks the trap mostly dry.
A plumber used a camera on the drain and said it is fine and suggested some interior drain vents with check valves. Could this fix the problem and why does it act up when it is raining hard?
I should have included that the contents of the septic/holding tank is pumped up into the new city sewer pipe.

[hj]

I cannot imagine ANY city allowing a hybrid system like that. Usually the septic system has to be filled with material and abandoned when the connection to the city is made. In addition you are probably paying a sewer charge for the water used by that toilet even though it is not going to the sewer. There can be many reasons for you situation, one of them being that the septic effluent field becomes saturated during rainy seasons and then the excess water ENTERS the tank and its piping causing a positive pressure in your piping, especially if it is not vented, and thus the bubbles.

[bicycler]

The work was done just before I moved in, but I understand the old laterals from the septic were disconnected and plugged, but I supposed water could be getting in to the septic tank somehow. Maybe around where the 24" plastic tile connects to the concrete tank part? I put and extension on the original plastic tile and sealed the crap out of that part so I don't think it is leaking there? If there is water getting into the septic/holding tank and then pumped into the sewer system I am sure the city would not be happy about that, but would the check valve that allows air into the sewer (that the local plumber is suggesting) do any good or would I need an outside open vent on the main sewer?

[bicycler]

I noticed the toilet bubbling today and it has not rained in a few days and no one had used the toilet recently. Now I suspect that this bubbling could be happening when ever the pump in the old septic tank turns on and empties the septic tank contents into the new sewer line. I guess I will just try installing some sort of vent in the line?

[hj]

How about NOT guessing and have a "real" plumber check your system to be sure it is operating legally. A lot of what you are saying does not come across as being logical. Putting vents in just because you "think they would cure the problem" can be a waste of time and money, especially if they are not installed properly.

[Gary Swart]

As HJ stated, you need a real plumber to evaluate you drain system. It would appear that you have more than a DIY situation here, and the person advising you on the AAVs is not a qualified plumber. I tell you that because not only will most (nearly all) plumbers veto the air admittance valves, aka Studor vents, one of the basic requirement of their use is that there is at least one vent to the outside and any licensed plumber should know that.