1987 Cressida

i have a 1987 toyota cressida. whenever i turn on the heater, the car is already heated up by the way, there is no heat coming out of it automatically. i have to play with the buttons in order for me to get heat. there are times that i do get heat but when i brake hard the heat turns to cold air. and now lately my car has no heat at all. even when i play with the buttons i get no heat..what can be the cause of this? the car by the doesnt overheat or anything.. it warms up normally..

Low coolant level
Faulty heater valve
Clogged or restricted heater core
Blend door not opening