The answer will depend on the language in your purchase contract and your local disclosure laws. Did you have an inspection contingency? Have you released that contingency yet? Also, in some states, the seller is required to fill out a property disclosure. Did they disclose that they had plumbing problems? Did they know about the problems and not disclose them to you?
A $10K repair bill is nothing to sneeze at. Talk to your agent about the specifics of your contract and what your rights and obligations might be. Most importantly, talk to an attorney - I bet you have more footing than you think you might and the small amount you'll pay for a lawyer will be well worth your piece of mind.