Hi, I'm trying to help my son finish his basement on a new house (built 1 year ago). 3 stubs were plumbed for the basement bathroom. Our initial activities: 1. Shower drain was capped (no shower being put in) 2. Sink drain was cut to place vanity (temporarily taped top until final plumbing and keep drain flies in) 3. Toilet drain was cut to install toilet after flooring. PVC was 3 inch schedule 30, so used an inside 3 inch twist in flange so floor would not need to be busted up. Toilet was installed. First flush was great. Subsequent flushes don't go down, water just rises in bowl. Bowl water eventually goes down after several minutes. A week later (we work on it on the weekends) we got around to finishing the plumbing for the sink. We left toilet issue alone, as we thought it might be a venting issue, which we hoped to resolve by installing studor value on the sink for venting. Tested toilet again and same as last time. First flush great again. Subsequent flushes don't evacuate water from bowl. Water just rises in bowl. Recedes to normal bowl water level after several minutes. Ran water in newly plumbed sink, and after a couple minutes, sink lines back up and water begins actually rising in the toilet next to the sink. Does this indicate a clogged line, or still a venting issue? In another section of the basement, we installed a kitchen area with sink, and plumbed that into a floor drain. We are having no issues at all with the water draining for the kitchen sink. Any next suggestions?