A 1985 4/4 will have the split circuit master cylinder , these has a central seal which can (and does) wear which leads to a very soft pedal and can occur suddenly. However this would not account for the fluid loss , so I would look elsewhere.
Again a 4/4 of that era would not have a servo as standard , so unless a previous owner has fitted one , I would discount that.
The question is when did you last check the fluid , ie: is it one sudden loss , or a gradual seeping away? That would point the investigation into 2 different paths.
If the former I would be looking for a brake pipe that is rubbing on the chassis , or a failed flexible hose perhaps, the fluid loss may not be evident under the car as it might be "held" on a chassis rail.
If the latter then I would regard the rear wheel cylinders as the likely culprit. As others have commented it is possible that the fluid seepage has been contained within the drum, and is not evident on the floor or under the car , but you need to pull the drums off and investigate.
