Coolant recovery

I have a 1974 Duster. I believe this was the first year they came equipped with the coolant recovery system. I also still have the factory 26" a/c radiator. No problems over-heating but a question regarding the yellow container to the left of the radiator. I always thought the purpose was to add coolant and if the radiator was low it would suck that extra coolant in. I have a small leak so from time to time I have been adding some to the container and once I was low that extra fluid would find it's way into the engine. I took the car out today but didn't add any, the container was almost empty. When I got back I looked in it and it was almost full of boiling coolant. A couple hours later I checked it again and it must have all gone back in because it was almost empty. Is it normal for fluid to be pushed out of the radiator when hot?? I was surprised to see that. Before I drove I also popped the radiator cap off and it was about full so I didn't feel the need to add some to the yellow container. Is this supposed to happen or is something wrong?

Thanks for any info.