Hi
1) The Settlement deed executed by your father is valid only after his time.
2) Your father has consciously retained life interest in the property and as such your father can cancel the settlement agreement executed by him.
3) Also your father is in possession of the property.
4) During his life time, your father can cancel the settlement agreement.
5) Now, in order to cancel the settlement agreement, you will need to cancel the settlement agreement through court given that
a) the settlement deed was executed in the year 1985 and
b) Unilateral cancellation of deed of conveyance can be done only through courts.
If your father is a senior citizen, then he can file for cancellation of settlement deed through senior citizens act.
Hope this information is useful.