If the coolant leaves the radiator when the engine warms anyway, why do you need a recovery system?
Technically, it operates as an expansion tank. The idea is to always have the radiator 100% full of coolant (no air) at any temperature. That reduces the likelihood of mixing air bubbles and circulating them in the system.
As far as the cap goes, the cap for a closed system actually will have two seals. The first seal is on the spring-loaded plunger which seals down in the filler neck. The other one, usually a simple gasket, seals the cap to the TOP of the filler neck.
The radiator's overflow outlet is located between these two seals.
- When system pressure builds, the plunger is pushed open and coolant goes past the lower seal and out the overflow tube.
- When the system cools down, low pressure pulls the coolant back into the radiator. Without the upper gasket, fluid will still go out the overflow tube since it is lower than the top of the filler neck, but when it cools back down, air is easily sucked through this connection since it isn't sealed. That's why with a recovery system, you don't open the cap to check the level but instead check it in the overflow tank - you don't want to break that seal and allow air into the radiator.
Back in the late 60's and early 70's, coolant recovery tank kits were a popular aftermarket item. The kits consisted of a tank, some hose
and a gasket to add to your old radiator cap. Once everything is in place, it will takes a few heat/cool cycles to totally purge all the air from your cooling system.
