I am NOT an attorney, so do NOT construe this as legal advice, but my understanding is that the seller may be liable if he both knew about the plumbing problems AND did not disclose them to you and you can prove it.
You should go through your file of the contract and closing to see if there is a seller's disclosure. If you can not find it contact the broker that your agent worked for. It is the broker that is the record keeper, not the agent and the broker maybe able to provide you a copy of the transaction file.
Did your next door neighbors know the seller? You may want to casually ask your neighbors if the seller had plumbing problems when they lived there. I say casually because if they liked their neighbor, they may not be as forcoming with information.