Legal Question in Real Estate Law in California

The sellers are probably right....at least insofar as a "duty to disclose" is concerned, there is a rebuttable presumption that someone who didn't live in a house would not know about a defect of this kind. There is no duty (and no possibility) to disclose what you don't know. You might want to inquire about the contractor that did the faulty work and see if there was any express or implied warranty. Properly-done plumbing doesn't leak after six months.

The sellers have a duty to disclose known defects, but the issue in your case is whether the sellers knew of the leak when they didn't live in the house. You are going to have to do some investigation, such as talking to any former tenants, and finding out if a repair was made by a contractor, as pointed out by Mr. Whipple.