Legal Question in Real Estate Law in California

Unfortunately the answer is almost certainly "yes, you are liable." Plumbing leaks are generally treated as "strict liability" torts, meaning that there is no requirement to prove breach of any duty of due care, which is the hallmark of negligence. If you own the plumbing and it causes damage to someone else's property, you are simply liable, period. If there is a dispute about what the actual cause was, or the other party failed to mitigate their damages, you can defend in whole or part on that basis in a proper case, but the basic liability is automatic.