The reality is that real estate is not a science. Aaron is correct that every situation is different and issues like these need to be handled on a case by case basis.
In practicality an argument can be made that little or no value was lost. You need to consider how buyer's typically buy homes to understand why. In a typical showing, property disclosures are rarely reviewed and almost without fail inspections aren't conducted until long after a buyer has fallen emotionally in love with a home. Most buyers are willing to look past many large defects as long as a couple of items fall in place - the issue is forthwritely disclosed, the problem was remedied, and the problem hasn't presisted.
In your case, if you fix the root cause of the problem, disclose the issue when you sell, and haven't had a problem since. It is likely the buyer will overlook the issues and assume that it has been resolved.
However, the question behind your question seems to be what can you go after the seller for. That is something your attorney will need to outline for you. To make your argument of devalution, have an appraiser come in, present the scenario to them, and ask them to present you with an appraisal assuming that the issues didn't exist and an appraisal now that you know that it does. The difference in the two will obviously be the amount of of devaluation. I had a client experience the same issue with a sewer backup and we had to hire an appraiser to conduct a similar appraisal for a lawsuit. Expect to pay a bit more for this more in depth appraisal. Good Luck!!
Coldwell Banker Burnet
licensed MN Broker