Starting a BB Duster project

Fixing the thermostat on the Duster also fixed the coolant recovery system. I had noticed that the level in the coolant recovery system was never changing and that seemed really odd. Now that the engine is coming up to temp the coolant recovery system is working. When I bring the car back in from a drive the bottle is full but after several hours the bottle is down near the low level. So the hot water is expanding and pushing into the bottle then as the coolant cools back down it shrinks and pulls in fluid from the bottle. I'm not sure why it wasn't working before since the water was getting hot. Maybe there was some sort of air bubble in the system?