This is a very unusual problem, and I would imagine your inspection did not uncover the "size" of the pipes used. I know it seems extreme, but it's very possible that more of the "work" is poor quality and you have to protect yourself now and hire a Real Estate attorney before it goes any further.
IF I REMEMBER CORRECTLY YOU HAVE 1 YEAR FROM DISCOVERY TO FILE THIS CLAIM, YOU MUST 1ST GIVE THE OWNER A DEMAND LETTER TO CORRECT THE WORK.
IT WOULD BE ADVISED TO OBTAIN ESTIMATES FOR ALL CORRECTION FROM A LICâ€™D CONTRACTOR SPELLING OUT THE DEFECTS, AND COSTâ€™S, AND USUALLY THE COURT LIKES TO SEE MORE THEN ONE ESTIMATE. NOTE: MANY TIMES A CONTRACTOR WILL DISCOVER OTHER DEFECTS OR CODE VIOLATIONS.
YOU MAY WANT TO GO TO THE CITY BUILDING DEPT. AND ASK TO REVIEW ANY BUILDING PERMITS TAKEN ON THE PROPERTY. THIS MAY FURTHER ESTABLISH YOUR CASE AGAINST THE OWNER IF NO PERMITS WHERE TAKEN.
IT IS NOT YOUR RESPONSIBILITY, BUT IF THE SELLER HIRED A LIC'D CONTRACTOR, HE MAY HAVE A CLAIM AGAINST THE CONTRACTOR TO REMEDY THESE DEFECT'S, BUT THAT IS NOT YOUR CONCERN NOR RESPONSIBILITY.
I AM NOT GIVING YOU LEGAL ADVICE BUT ONLY A SUGGESTION.