I live in an old house with old pipes. I noticed one of my toilets when flushed would just fill up with water and then very slowly drain. The toilets were old so we replaced the toilet thinking this would solve the problem but it did not. The new toilet does the same thing. Now when I turn on the sink in the same bathroom I can hear the toilet bubbling. There is an adjacent bathroom and the toilet flushes just fine. Any ideas on how to solve this problem?

Maybe a plugged, capped or inadequate vent line is causing a vacuum which draws water back out of the toilet in question.

I have one that most repair plumbers miss.. Look down at the bottom of the bowl. If there is a small hole, then that is a jet that starts the syphon action. If it's clogged the water will just swirl around and slowly go down leaving solids behind. Take your finger,(UGH! ) and run it around the inside of the opening. Over the years minerals build up and cut down on the syphon (flush) action. If it is rough or you feel build up, take a table knife and put a bend in it to get around the curve in the bowl and chip and scrap it clear. Next take a coathanger and clear out the holes around the rim. They start the swirling action. And last, check the water level in the tank. It should be 1/2" below the top of the over flow tube. And speaking of the overflow tube, Make sure the small 1/8" tube from the ballcock to the white overflow tube is connected so it discharges in it and that it's flowing when the ballcock fills. This is what raises the water level in the bowl. For a good solid flush they all have to work together. Hope this helps. Good luck, Tom