is it possible to add vent to drain/waste system

We bought an old house that originally did not have a bathroom in it. Sometime in the 50s they added a bathroom & kitchen. There is a very small vent pipe on the outside wall of the kitchen, no vent for the bathroom. After purchasing the house we had a 2nd bathroom installed. I never really paid attention to the fact that there was no vent put in for the 2nd bath.

I started renovating the kitchen. Right now I have the drain for the old kitchen sink capped off until i get the new floor & cabinets installed in the kitchen.

Recently when using the shower, the toilet "gurgles". Then over the weekend my girlfriend was doing a load of laundry & when the washer drained it backed up through the shower drain.

I went outside to the drain clean out that is between the house & the city sewer, well when i unscrewed the cap I had a sewer eruption...
Not pretty to say the least...

I snaked out the drain heading towards the city sewer & after about 10-15 minutes all of my waste water had drained to the sewer & I had less than a 1/4 inch of waste water in the bottom of the pipe & it was flowing to the sewer.

Now it is 2 days later & I took a shower & the toilet started gurgling again.

This is when I finally thought about there not being a vent anywhere except for outside the kitchen wall.

Am I correct in thinking that my problem may be that I do not have enough venting? And when I opened up the clean out it "vented" enough to allow the waste to drain out?

And if so can I add a vent? My new bathroom has the toilet at the end of the drain line, then it goes to the shower, can I cut in a Y and put a vent in between the toilet & shower?

Also, say i get another sewer back up, what keeps the waste water from backing up thru the vent? Is it just a case of the shower drain or whatever is lower than the top of the vent?

Hello Racerboy and Welcome to the Community:
The lack of vents will certainly make some drains gurgle but it would never cause tha blockage in the sewer.
There may be a maple tree that has found a leak in the sewer and is going into the sewer for the water and natural fertilizer. They will stop a drain in a hurry and, when you snake the drain, you may not be getting all of the blockage back. Some of it goes on into the city sewer.
Another notorious blocker is bermuda grass. It has a root system 3' deep or more and will invade a sewer for water; rapidly growing roots into the sewer to pick up water and food.
One easy fix for the sewer blockage is to get a bag of the water softener salt pellets and flush a cup of them every week. They are large enough to become tangled in any roots and will lay there and feed salt to the roots, which keeps them killed.
If your sewer is vitrified clay pipe it is good for a lifetime or two; it just gets cracks in the joints and allows roots in. If it is Orangeburg pipe (which is made of roofing felt and tar in layers) prepare to have it replaced. It can collapse completely.
There is a new innovation where they dig up both ends of the sewer and run a cable through it, attach a cutter head and new pipe to it and pull the new pipe through the old without distrubing the surface, driveways, flowerbeds and such.
Yes, more vents would help the water to drain quicker but I don't think that is your problem. If you can open a small place in the wall behind the bath sink, take the trap and exposed pipe off, remove the ell inside the wall and replace it with a tee. There is a mechanical vent that is illegal anywhere except in trailers, that can be screwed into the top of the tee. It is easier than trying to run a new vent pipe and cutting into the house drain while costing quite a bit less. You may have to go to a Mobile Home parts place to get one of those vents but one in each bathroom would relieve some of your problems.
Glenn

thanks for your reply...
i was getting ready to tackle this job this weekend...
We have what you call "orangeburg" pipe running from the house to the street... Guess I will be digging up the yard this weekend....
I may eventually add a vent to the newest bathroom, but right now I will focus on getting that pipe replaced