I thought I had fixed the issue by adding an additional air intake to the line but the other day the tenant drained the washer and then water was pouring out of the sink/drain, I'm wondering if it's because there may be a clog where the new pvc wastlines from the kitchen, meet an existing 2" cast iron pipe which meets the 4" wasteline that the bathrooms connect to. I haven't had any problems with the bathrooms. Does this sound plausible? if not then would adding yet another air intake to the line but after the sink waste, correct this issue? or do I need to take out the existing cast iron and put in more pvc?