We bought our home back in March,that has septic well, and redid the bathroom. We added a new toilet, bath/shower, and vanity. We kept the plumbing original. Now when we flush the toilet it bubbles, and either the sink, and or the tub gurgle. I went to the top of the vent pipe and poured water and it drains very very slowly. Half of the piping is original(looks like galvanized steel vent pipe to basement)and the rest is PVC (done by previous owner) I tried snaking it but couldn't get past the first 90 from the top of the vent stack. I am thinking it is blocked but the upright stack goes from a 2" to 1 1/2"with a 90 reducer, could this also be a problem? Should I replace the piping?

when we flush the toilet it bubbles, and either the sink, and or the tub gurgle. I went to the top of the vent pipe and poured water and it drains very very slowly.The bubble indicates a back pressure or partial blocking's occurring. The gaggles plus the fact that there should be no water build up in a vent pipe tells me that you have a blocked vent.
It's time to call in a pro to let you know what it will take to bring everything back to working order,
Good luck, Tom

U have blockage down stream from toilet. Try snaking through toilet. If this does not work, remove toilet and snake from there.

Before going to bed at night, pour 2 cups of vinegar in drain. In morning pour 2 gallons boiling water down drain.

Have u had septic tank cleaned since u bought? This does not appear to be problem, but should be done if u don't know when it was last cleaned.

Chuck

The septic was recently cleaned (just before we bought the house) I did clean the pipe before the install, and was very careful not to get debri down the holes. I figured it might require new piping since the galvanized was corroded on the inside, and the fact it goes from a 2"to 1 1/2" 90 degree bend to just. Oh well thank you to everyone this has been an exceptionally helpful site!