The only thing you need to find out is the value of the land in 1986, since land is not depreciated. Use a tax bill, or assesment, or speak with a realtor (one that's been around a while) to give you an estimate of the land value in 1986.
The building would have been completely depreciated by now since renat have a 27.5 year life. How you depreciated it without a value is a mystery to me, but whatever you used, it's zero now.
Even if you did not take the depreciation expenses on the rental, the IRS has you re-pay the depreciation that was taken, or could have been taken.
So the remaining basis in the property is the original 1986 value of the land, and the remaining 17,659 for the septic. The rest is long-term capital gains.
