The country house I live in was built 40+ years ago. Had septic tank replaced and a broken exit pipe repaired. Now air is trapped in the house pipes so upstairs drains into the downstairs tub and the downstairs toilet will not evacuate. Discovered there is no plumbing vent. Am thinking AAVs might help but that won't evacuate gases. Essentially I cannot use any showers, toilets or dishwasher until this is resolved. Need some suggestions.
If you were to remove a p-trap from under a sink upstairs, you'd basically be opening a vent. If you do this I doubt you'd notice any change however. You sound like you have a blockage in your drain line. Lack of a vent isn't going to force your entire drain to hold water like that, it would just prevent a full volume of drain flow from occuring. You need to snake your mainline, the line leading into your septic has a blockage somewhere. That is your actual issue.
It would help to know the drain line layout. That would allow for specific direction. At a minimum you will need a 2" vent line at the highest point in the drain system. The vent should be routed through the roof. If you have more than one high point, each high point must have its own vent routed into the attic. Vent lines may be joined together within the walls or in the attic area, or they may be individually run through the roof. If joined to a common vent, the common vent must be 3" or larger. Adding vent lines will require opening walls and reinforcing top/bottom plates and some studs. But that's what we get with older homes. Note: a "high point" is the highest point in a single drain line or group of connected drain lines.