Most items don't "survive" closing - which means if it's working at the walk through - seller isn't responsible following the cosing if it breaks.
Now, if there was fraud invovled, that's a different story.
If the sellers were told the tank "was bad" 4 years ago, and they continued to keep it as it was, apparently they didnt agree with the contractor.
The biggest question I have is - Did you have it inspected prior to closing???
If so.............What did your inspection indicate?
Did your inspector perhaps miss something? You night want to give him a call, too.
Unless you can show that the sellers had current issues that they HID from you, and didn't disclose..........they or misrepresented something - I am not sure what recourse you have.
You need legal advice....call an attorney.
You and really should have had a Septic inspection.
If the Septic guy will testify that the owner knew......
You should talk to a lawyer, at this point.
If other people are seeing this, please get those inspections; it wll save more than they cost!
The current owners may never have had an issue, they may have been unaware that there were problems with the system. The contractor who came out, might be mistaken about who he talked to before or the owners you bought from, might not be the same owners he talked to. Bottom line, typically the buyer has to PROVE, that the seller knew and purposely did not tell the buyer. You have to bear the expense of fixing the problem and taking them to court. NOW, if your home owners insurance and/or your home warranty cover the issues, USE THEM. Then let them discuss the issues with the sellers insurance company or seller.
Document, document, document.....begin with getting a statement from the septic company that supports the information that he shared with you. Get a second/ third...opinion on the nature of the work necessary to put the system in working order and speak with an atttorney about your options.
Often the threat of possible legal action in the form of a letter from an attorney can help people to see things more clearly without having to take it to a higher level.
Unfortunately, it may come down to making a decision based on the expense of making the repairs or paying the legal fees to persue it further.....with no guarentees!
Begin by collecting your information, consulting an attorney, and submitting a letter via the attorney.