I am building a new house and the plumber inserted a plug of some type in the sewer line when he did the rough-in.

The plumber has gone away and we have just received our Occupancy Permit and now I need to remove the plug to activate the sewer line. Question: How do I remove this plug? The plug has a small nylon cord that comes up to a float at the sewer clean out. I am afraid to pull too hard and break the cord.