Hi all. I have a 2 inch drain going into the mainstack via a short run of about 1foot of 1 1/2 inch. There is a slop sink at the point where the pipe reduction is. Is there a problem with this drain backing up because of the restricted flow of the downsized pipe? The fitting on the stack is 1 1/2 threaded cast iron. Would it be best to open this up to 2 inch as close to the stack as possible? Thanks in advance.