My parents' toilet will appear "plugged" however plunging it or snaking it never finds anything or helps. If you wait long enough, it will over time very slowly drain. We have disconnected the pipe in the basement between the cast iron part that contains the roof vent and the PVC straight run that heads to the septic tank. When this is disconnected and you pour a bucket of water into the toilet upstairs it runs out quickly. Following reassembly everything seems fine for a few weeks(?) then it will happen again. It seems like breaking the pipe joint in the drain seems to "fix" it. Is it getting an airlock from somewhere? If so where and how might we test?

Thanks.

I had these exact symptoms and found out only after an amazingly long and frustrating series of events, that I will save you the details of here, that the snaking needed to happen through the actual roof vent so that it could effectively get to the problem area and do the trick. Just fyi.

You will need to rod your vent stack from the roof. Use enough rod to reach roof to basement and then 15 more feet. The vent is designed to allow air to be pushed out of the drain pipe when water rushes in, I think your vent is plugged.